Add support for multiple readers (#2596)

src/OpenTelemetry/.publicApi/net461/PublicAPI.Unshipped.txt

@@ -121,7 +121,6 @@ static OpenTelemetry.Metrics.MetricTypeExtensions.IsGauge(this OpenTelemetry.Met
 static OpenTelemetry.Metrics.MetricTypeExtensions.IsHistogram(this OpenTelemetry.Metrics.MetricType self) -> bool
 static OpenTelemetry.Metrics.MetricTypeExtensions.IsLong(this OpenTelemetry.Metrics.MetricType self) -> bool
 static OpenTelemetry.Metrics.MetricTypeExtensions.IsSum(this OpenTelemetry.Metrics.MetricType self) -> bool
-static OpenTelemetry.ProviderExtensions.GetMetricCollect(this OpenTelemetry.BaseProvider baseProvider) -> System.Func<OpenTelemetry.Batch<OpenTelemetry.Metrics.Metric>>
 static OpenTelemetry.Sdk.CreateMeterProviderBuilder() -> OpenTelemetry.Metrics.MeterProviderBuilder
 static readonly OpenTelemetry.Metrics.MetricStreamConfiguration.Drop -> OpenTelemetry.Metrics.MetricStreamConfiguration
 virtual OpenTelemetry.BaseExporter<T>.OnForceFlush(int timeoutMilliseconds) -> bool

src/OpenTelemetry/.publicApi/netstandard2.0/PublicAPI.Unshipped.txt

@@ -121,7 +121,6 @@ static OpenTelemetry.Metrics.MetricTypeExtensions.IsGauge(this OpenTelemetry.Met
 static OpenTelemetry.Metrics.MetricTypeExtensions.IsHistogram(this OpenTelemetry.Metrics.MetricType self) -> bool
 static OpenTelemetry.Metrics.MetricTypeExtensions.IsLong(this OpenTelemetry.Metrics.MetricType self) -> bool
 static OpenTelemetry.Metrics.MetricTypeExtensions.IsSum(this OpenTelemetry.Metrics.MetricType self) -> bool
-static OpenTelemetry.ProviderExtensions.GetMetricCollect(this OpenTelemetry.BaseProvider baseProvider) -> System.Func<OpenTelemetry.Batch<OpenTelemetry.Metrics.Metric>>
 static OpenTelemetry.Sdk.CreateMeterProviderBuilder() -> OpenTelemetry.Metrics.MeterProviderBuilder
 static readonly OpenTelemetry.Metrics.MetricStreamConfiguration.Drop -> OpenTelemetry.Metrics.MetricStreamConfiguration
 virtual OpenTelemetry.BaseExporter<T>.OnForceFlush(int timeoutMilliseconds) -> bool

src/OpenTelemetry/CHANGELOG.md

@@ -28,28 +28,31 @@
 * Added wildcard support for AddMeter.
   ([#2459](https://github.com/open-telemetry/opentelemetry-dotnet/pull/2459))
 
+* Add support for multiple Metric readers
+  ([#2596](https://github.com/open-telemetry/opentelemetry-dotnet/pull/2596))
+
 ## 1.2.0-beta1
 
 Released 2021-Oct-08
 
-* Exception from Observable instrument callbacks does not
+* Exception from Observable instrument callbacks does not result in entire
-  result in entire metrics being lost.
+  metrics being lost.
 
-* SDK is allocation-free on recording of measurements with
+* SDK is allocation-free on recording of measurements with upto 8 tags.
-  upto 8 tags.
 
 * TracerProviderBuilder.AddLegacySource now supports wildcard activity names.
   ([#2183](https://github.com/open-telemetry/opentelemetry-dotnet/issues/2183))
 
-* Instrument and View names are validated
+* Instrument and View names are validated [according with the
-  [according with the spec](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md#instrument).
+  spec](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md#instrument).
   ([#2470](https://github.com/open-telemetry/opentelemetry-dotnet/issues/2470))
 
 ## 1.2.0-alpha4
 
 Released 2021-Sep-23
 
-* `BatchExportProcessor.OnShutdown` will now log the count of dropped telemetry items.
+* `BatchExportProcessor.OnShutdown` will now log the count of dropped telemetry
+  items.
   ([#2331](https://github.com/open-telemetry/opentelemetry-dotnet/pull/2331))
 * Changed `CompositeProcessor<T>.OnForceFlush` to meet with the spec
   requirement. Now the SDK will invoke `ForceFlush` on all registered
@@ -60,14 +63,14 @@ Released 2021-Sep-23
 
 Released 2021-Sep-13
 
-* Metrics perf improvements, bug fixes.
+* Metrics perf improvements, bug fixes. Replace MetricProcessor with
-  Replace MetricProcessor with MetricReader.
+  MetricReader.
   ([#2306](https://github.com/open-telemetry/opentelemetry-dotnet/pull/2306))
 
-* Add `BatchExportActivityProcessorOptions` which supports field value overriding
+* Add `BatchExportActivityProcessorOptions` which supports field value
-  using `OTEL_BSP_SCHEDULE_DELAY`, `OTEL_BSP_EXPORT_TIMEOUT`,
+  overriding using `OTEL_BSP_SCHEDULE_DELAY`, `OTEL_BSP_EXPORT_TIMEOUT`,
-  `OTEL_BSP_MAX_QUEUE_SIZE`, `OTEL_BSP_MAX_EXPORT_BATCH_SIZE`
+  `OTEL_BSP_MAX_QUEUE_SIZE`, `OTEL_BSP_MAX_EXPORT_BATCH_SIZE` envionmental
-  envionmental variables as defined in the
+  variables as defined in the
   [specification](https://github.com/open-telemetry/opentelemetry-specification/blob/v1.5.0/specification/sdk-environment-variables.md#batch-span-processor).
   ([#2219](https://github.com/open-telemetry/opentelemetry-dotnet/pull/2219))
 
@@ -75,21 +78,24 @@ Released 2021-Sep-13
 
 Released 2021-Aug-24
 
-* More Metrics features. All instrument types, push/pull
+* More Metrics features. All instrument types, push/pull exporters,
-  exporters, Delta/Cumulative temporality supported.
+  Delta/Cumulative temporality supported.
 
-* `ResourceBuilder.CreateDefault` has detectors for
+* `ResourceBuilder.CreateDefault` has detectors for `OTEL_RESOURCE_ATTRIBUTES`,
-  `OTEL_RESOURCE_ATTRIBUTES`, `OTEL_SERVICE_NAME` environment variables
+  `OTEL_SERVICE_NAME` environment variables so that explicit
-  so that explicit `AddEnvironmentVariableDetector` call is not needed. ([#2247](https://github.com/open-telemetry/opentelemetry-dotnet/pull/2247))
+  `AddEnvironmentVariableDetector` call is not needed.
+  ([#2247](https://github.com/open-telemetry/opentelemetry-dotnet/pull/2247))
 
 * `ResourceBuilder.AddEnvironmentVariableDetector` handles `OTEL_SERVICE_NAME`
-   environmental variable. ([#2209](https://github.com/open-telemetry/opentelemetry-dotnet/pull/2209))
+   environmental variable.
+   ([#2209](https://github.com/open-telemetry/opentelemetry-dotnet/pull/2209))
 
-* Removes upper constraint for Microsoft.Extensions.Logging
+* Removes upper constraint for Microsoft.Extensions.Logging dependencies.
-  dependencies. ([#2179](https://github.com/open-telemetry/opentelemetry-dotnet/pull/2179))
+  ([#2179](https://github.com/open-telemetry/opentelemetry-dotnet/pull/2179))
 
-* OpenTelemetryLogger modified to not throw, when the
+* OpenTelemetryLogger modified to not throw, when the formatter supplied in
-  formatter supplied in ILogger.Log call is null. ([#2200](https://github.com/open-telemetry/opentelemetry-dotnet/pull/2200))
+  ILogger.Log call is null.
+  ([#2200](https://github.com/open-telemetry/opentelemetry-dotnet/pull/2200))
 
 ## 1.2.0-alpha1
 
@@ -100,7 +106,8 @@ Released 2021-Jul-23
   ([#2174](https://github.com/open-telemetry/opentelemetry-dotnet/pull/2174))
 
 * Removes .NET Framework 4.5.2, .NET 4.6 support. The minimum .NET Framework
-  version supported is .NET 4.6.1. ([#2138](https://github.com/open-telemetry/opentelemetry-dotnet/issues/2138))
+  version supported is .NET 4.6.1.
+  ([#2138](https://github.com/open-telemetry/opentelemetry-dotnet/issues/2138))
 
 ## 1.1.0
 
@@ -137,8 +144,8 @@ Released 2021-May-11
 Released 2021-Apr-23
 
 * Use `AssemblyFileVersionAttribute` instead of `FileVersionInfo.GetVersionInfo`
-  to get the SDK version attribute to ensure that it works when the assembly
+  to get the SDK version attribute to ensure that it works when the assembly is
-  is not loaded directly from a file on disk
+  not loaded directly from a file on disk
   ([#1908](https://github.com/open-telemetry/opentelemetry-dotnet/issues/1908))
 
 ## 1.1.0-beta1

src/OpenTelemetry/Metrics/CompositeMetricReader.cs

@@ -22,11 +22,15 @@ using OpenTelemetry.Internal;
 
 namespace OpenTelemetry.Metrics
 {
-    internal sealed class CompositeMetricReader : MetricReader
+    /// <summary>
+    /// CompositeMetricReader that does not deal with adding metrics and recording measurements.
+    /// </summary>
+    internal sealed partial class CompositeMetricReader : MetricReader
     {
         private readonly DoublyLinkedListNode head;
         private DoublyLinkedListNode tail;
         private bool disposed;
+        private int count;
 
         public CompositeMetricReader(IEnumerable<MetricReader> readers)
         {
@@ -40,6 +44,7 @@ namespace OpenTelemetry.Metrics
 
             this.head = new DoublyLinkedListNode(iter.Current);
             this.tail = this.head;
+            this.count++;
 
             while (iter.MoveNext())
             {
@@ -57,6 +62,7 @@ namespace OpenTelemetry.Metrics
             };
             this.tail.Next = node;
             this.tail = node;
+            this.count++;
 
             return this;
         }

src/OpenTelemetry/Metrics/CompositeMetricReaderExt.cs

@@ -0,0 +1,149 @@
+// <copyright file="CompositeMetricReaderExt.cs" company="OpenTelemetry Authors">
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+// </copyright>
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Diagnostics.Metrics;
+
+namespace OpenTelemetry.Metrics
+{
+    /// <summary>
+    /// CompositeMetricReader that deals with adding metrics and recording measurements.
+    /// </summary>
+    internal sealed partial class CompositeMetricReader
+    {
+        internal List<Metric> AddMetricsWithNoViews(Instrument instrument)
+        {
+            var metrics = new List<Metric>(this.count);
+            for (var cur = this.head; cur != null; cur = cur.Next)
+            {
+                var metric = cur.Value.AddMetricWithNoViews(instrument);
+                metrics.Add(metric);
+            }
+
+            return metrics;
+        }
+
+        internal void RecordSingleStreamLongMeasurements(List<Metric> metrics, long value, ReadOnlySpan<KeyValuePair<string, object>> tags)
+        {
+            Debug.Assert(metrics.Count == this.count, "The count of metrics to be updated for a CompositeReader must match the number of individual readers.");
+
+            int index = 0;
+            for (var cur = this.head; cur != null; cur = cur.Next)
+            {
+                if (metrics[index] != null)
+                {
+                    cur.Value.RecordSingleStreamLongMeasurement(metrics[index], value, tags);
+                }
+
+                index++;
+            }
+        }
+
+        internal void RecordSingleStreamDoubleMeasurements(List<Metric> metrics, double value, ReadOnlySpan<KeyValuePair<string, object>> tags)
+        {
+            Debug.Assert(metrics.Count == this.count, "The count of metrics to be updated for a CompositeReader must match the number of individual readers.");
+
+            int index = 0;
+            for (var cur = this.head; cur != null; cur = cur.Next)
+            {
+                if (metrics[index] != null)
+                {
+                    cur.Value.RecordSingleStreamDoubleMeasurement(metrics[index], value, tags);
+                }
+
+                index++;
+            }
+        }
+
+        internal List<List<Metric>> AddMetricsSuperListWithViews(Instrument instrument, List<MetricStreamConfiguration> metricStreamConfigs)
+        {
+            var metricsSuperList = new List<List<Metric>>(this.count);
+            for (var cur = this.head; cur != null; cur = cur.Next)
+            {
+                var metrics = cur.Value.AddMetricsListWithViews(instrument, metricStreamConfigs);
+                metricsSuperList.Add(metrics);
+            }
+
+            return metricsSuperList;
+        }
+
+        internal void RecordLongMeasurements(List<List<Metric>> metricsSuperList, long value, ReadOnlySpan<KeyValuePair<string, object>> tags)
+        {
+            Debug.Assert(metricsSuperList.Count == this.count, "The count of metrics to be updated for a CompositeReader must match the number of individual readers.");
+
+            int index = 0;
+            for (var cur = this.head; cur != null; cur = cur.Next)
+            {
+                if (metricsSuperList[index].Count > 0)
+                {
+                    cur.Value.RecordLongMeasurement(metricsSuperList[index], value, tags);
+                }
+
+                index++;
+            }
+        }
+
+        internal void RecordDoubleMeasurements(List<List<Metric>> metricsSuperList, double value, ReadOnlySpan<KeyValuePair<string, object>> tags)
+        {
+            Debug.Assert(metricsSuperList.Count == this.count, "The count of metrics to be updated for a CompositeReader must match the number of individual readers.");
+
+            int index = 0;
+            for (var cur = this.head; cur != null; cur = cur.Next)
+            {
+                if (metricsSuperList[index].Count > 0)
+                {
+                    cur.Value.RecordDoubleMeasurement(metricsSuperList[index], value, tags);
+                }
+
+                index++;
+            }
+        }
+
+        internal void CompleteSingleStreamMeasurements(List<Metric> metrics)
+        {
+            Debug.Assert(metrics.Count == this.count, "The count of metrics to be updated for a CompositeReader must match the number of individual readers.");
+
+            int index = 0;
+            for (var cur = this.head; cur != null; cur = cur.Next)
+            {
+                if (metrics[index] != null)
+                {
+                    cur.Value.CompleteSingleStreamMeasurement(metrics[index]);
+                }
+
+                index++;
+            }
+        }
+
+        internal void CompleteMesaurements(List<List<Metric>> metricsSuperList)
+        {
+            Debug.Assert(metricsSuperList.Count == this.count, "The count of metrics to be updated for a CompositeReader must match the number of individual readers.");
+
+            int index = 0;
+            for (var cur = this.head; cur != null; cur = cur.Next)
+            {
+                if (metricsSuperList[index].Count > 0)
+                {
+                    cur.Value.CompleteMeasurement(metricsSuperList[index]);
+                }
+
+                index++;
+            }
+        }
+    }
+}

src/OpenTelemetry/Metrics/MeterProviderBuilderBase.cs

@@ -71,11 +71,6 @@ namespace OpenTelemetry.Metrics
 
         internal MeterProviderBuilder AddReader(MetricReader reader)
         {
-            if (this.MetricReaders.Count >= 1)
-            {
-                throw new InvalidOperationException("Only one Metricreader is allowed.");
-            }
-
             this.MetricReaders.Add(reader);
             return this;
         }

src/OpenTelemetry/Metrics/MeterProviderSdk.cs

@@ -27,18 +27,13 @@ namespace OpenTelemetry.Metrics
 {
     internal sealed class MeterProviderSdk : MeterProvider
     {
-        internal const int MaxMetrics = 1000;
         internal int ShutdownCount;
-        private readonly Metric[] metrics;
-        private readonly Metric[] metricsCurrentBatch;
         private readonly List<object> instrumentations = new List<object>();
         private readonly List<Func<Instrument, MetricStreamConfiguration>> viewConfigs;
         private readonly object collectLock = new object();
-        private readonly object instrumentCreationLock = new object();
-        private readonly HashSet<string> metricStreamNames = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
         private readonly MeterListener listener;
         private readonly MetricReader reader;
-        private int metricIndex = -1;
+        private readonly CompositeMetricReader compositeMetricReader;
         private bool disposed;
 
         internal MeterProviderSdk(
@@ -50,10 +45,6 @@ namespace OpenTelemetry.Metrics
         {
             this.Resource = resource;
             this.viewConfigs = viewConfigs;
-            this.metrics = new Metric[MaxMetrics];
-            this.metricsCurrentBatch = new Metric[MaxMetrics];
-
-            AggregationTemporality temporality = AggregationTemporality.Cumulative;
 
             foreach (var reader in readers)
             {
@@ -61,10 +52,6 @@ namespace OpenTelemetry.Metrics
 
                 reader.SetParentProvider(this);
 
-                // TODO: Actually support multiple readers.
-                // Currently the last reader's temporality wins.
-                temporality = reader.PreferredAggregationTemporality;
-
                 if (this.reader == null)
                 {
                     this.reader = reader;
@@ -79,6 +66,8 @@ namespace OpenTelemetry.Metrics
                 }
             }
 
+            this.compositeMetricReader = this.reader as CompositeMetricReader;
+
             if (instrumentationFactories.Any())
             {
                 foreach (var instrumentationFactory in instrumentationFactories)
@@ -107,6 +96,9 @@ namespace OpenTelemetry.Metrics
 
             this.listener = new MeterListener();
             var viewConfigCount = this.viewConfigs.Count;
+
+            // We expect that all the readers to be added are provided before MeterProviderSdk is built.
+            // If there are no readers added, we do not enable measurements for the instruments.
             if (viewConfigCount > 0)
             {
                 this.listener.InstrumentPublished = (instrument, listener) =>
@@ -141,75 +133,25 @@ namespace OpenTelemetry.Metrics
                         metricStreamConfigs.Add(null);
                     }
 
-                    var maxCountMetricsToBeCreated = metricStreamConfigs.Count;
+                    if (this.reader != null)
-
-                    // Create list with initial capacity as the max metric count.
-                    // Due to duplicate/max limit, we may not end up using them
-                    // all, and that memory is wasted until Meter disposed.
-                    // TODO: Revisit to see if we need to do metrics.TrimExcess()
-                    var metrics = new List<Metric>(maxCountMetricsToBeCreated);
-                    lock (this.instrumentCreationLock)
                     {
-                        for (int i = 0; i < maxCountMetricsToBeCreated; i++)
+                        if (this.compositeMetricReader == null)
                         {
-                            var metricStreamConfig = metricStreamConfigs[i];
+                            var metrics = this.reader.AddMetricsListWithViews(instrument, metricStreamConfigs);
-                            var meterName = instrument.Meter.Name;
-                            var metricName = metricStreamConfig?.Name ?? instrument.Name;
-                            var metricStreamName = $"{meterName}.{metricName}";
-
-                            if (!MeterProviderBuilderSdk.IsValidInstrumentName(metricName))
-                            {
-                                OpenTelemetrySdkEventSource.Log.MetricInstrumentIgnored(
-                                    metricName,
-                                    instrument.Meter.Name,
-                                    "Metric name is invalid.",
-                                    "The name must comply with the OpenTelemetry specification.");
-
-                                continue;
-                            }
-
-                            if (this.metricStreamNames.Contains(metricStreamName))
-                            {
-                                // TODO: Log that instrument is ignored
-                                // as the resulting Metric name is conflicting
-                                // with existing name.
-                                continue;
-                            }
-
-                            if (metricStreamConfig?.Aggregation == Aggregation.Drop)
-                            {
-                                // TODO: Log that instrument is ignored
-                                // as user explicitly asked to drop it
-                                // with View.
-                                continue;
-                            }
-
-                            var index = ++this.metricIndex;
-                            if (index >= MaxMetrics)
-                            {
-                                // TODO: Log that instrument is ignored
-                                // as max number of Metrics have reached.
-                            }
-                            else
-                            {
-                                Metric metric;
-                                var metricDescription = metricStreamConfig?.Description ?? instrument.Description;
-                                string[] tagKeysInteresting = metricStreamConfig?.TagKeys;
-                                double[] histogramBucketBounds = (metricStreamConfig is ExplicitBucketHistogramConfiguration histogramConfig
-                                    && histogramConfig.Boundaries != null) ? histogramConfig.Boundaries : null;
-                                metric = new Metric(instrument, temporality, metricName, metricDescription, histogramBucketBounds, tagKeysInteresting);
-
-                                this.metrics[index] = metric;
-                                metrics.Add(metric);
-                                this.metricStreamNames.Add(metricStreamName);
-                            }
-                        }
-
                             if (metrics.Count > 0)
                             {
                                 listener.EnableMeasurementEvents(instrument, metrics);
                             }
                         }
+                        else
+                        {
+                            var metricsSuperList = this.compositeMetricReader.AddMetricsSuperListWithViews(instrument, metricStreamConfigs);
+                            if (metricsSuperList.Any(metrics => metrics.Count > 0))
+                            {
+                                listener.EnableMeasurementEvents(instrument, metricsSuperList);
+                            }
+                        }
+                    }
                 };
 
                 // Everything double
@@ -247,33 +189,25 @@ namespace OpenTelemetry.Metrics
                             return;
                         }
 
-                        var meterName = instrument.Meter.Name;
+                        if (this.reader != null)
-                        var metricName = instrument.Name;
-                        var metricStreamName = $"{meterName}.{metricName}";
-                        Metric metric = null;
-                        lock (this.instrumentCreationLock)
                         {
-                            if (this.metricStreamNames.Contains(metricStreamName))
+                            if (this.compositeMetricReader == null)
                             {
-                                OpenTelemetrySdkEventSource.Log.MetricInstrumentIgnored(metricName, instrument.Meter.Name, "Metric name conflicting with existing name.", "Either change the name of the instrument or change name using View.");
+                                var metric = this.reader.AddMetricWithNoViews(instrument);
-                                return;
+                                if (metric != null)
+                                {
+                                    listener.EnableMeasurementEvents(instrument, metric);
                                 }
-
-                            var index = ++this.metricIndex;
-                            if (index >= MaxMetrics)
-                            {
-                                OpenTelemetrySdkEventSource.Log.MetricInstrumentIgnored(metricName, instrument.Meter.Name, "Maximum allowed Metrics for the provider exceeded.", "Use views to drop unused instruments. Or configure Provider to allow higher limit.");
-                                return;
                             }
                             else
                             {
-                                metric = new Metric(instrument, temporality, metricName, instrument.Description);
+                                var metrics = this.compositeMetricReader.AddMetricsWithNoViews(instrument);
-                                this.metrics[index] = metric;
+                                if (metrics.Any(metric => metric != null))
-                                this.metricStreamNames.Add(metricStreamName);
+                                {
+                                    listener.EnableMeasurementEvents(instrument, metrics);
+                                }
                             }
                         }
-
-                        listener.EnableMeasurementEvents(instrument, metric);
                     }
                     catch (Exception)
                     {
@@ -311,122 +245,163 @@ namespace OpenTelemetry.Metrics
 
         internal void MeasurementsCompletedSingleStream(Instrument instrument, object state)
         {
-            var metric = state as Metric;
+            Debug.Assert(instrument != null, "instrument must be non-null.");
-            if (metric == null)
+
+            if (this.compositeMetricReader == null)
+            {
+                if (state is not Metric metric)
                 {
                     // TODO: log
                     return;
                 }
 
-            metric.InstrumentDisposed = true;
+                this.reader.CompleteSingleStreamMeasurement(metric);
+            }
+            else
+            {
+                if (state is not List<Metric> metrics)
+                {
+                    // TODO: log
+                    return;
+                }
+
+                this.compositeMetricReader.CompleteSingleStreamMeasurements(metrics);
+            }
         }
 
         internal void MeasurementsCompleted(Instrument instrument, object state)
         {
-            var metrics = state as List<Metric>;
+            Debug.Assert(instrument != null, "instrument must be non-null.");
-            if (metrics == null)
+
+            if (this.compositeMetricReader == null)
+            {
+                if (state is not List<Metric> metrics)
                 {
                     // TODO: log
                     return;
                 }
 
-            foreach (var metric in metrics)
+                this.reader.CompleteMeasurement(metrics);
+            }
+            else
             {
-                metric.InstrumentDisposed = true;
+                if (state is not List<List<Metric>> metricsSuperList)
+                {
+                    // TODO: log
+                    return;
+                }
+
+                this.compositeMetricReader.CompleteMesaurements(metricsSuperList);
             }
         }
 
         internal void MeasurementRecordedDouble(Instrument instrument, double value, ReadOnlySpan<KeyValuePair<string, object>> tagsRos, object state)
         {
-            // Get Instrument State
-            var metrics = state as List<Metric>;
-
             Debug.Assert(instrument != null, "instrument must be non-null.");
-            if (metrics == null)
+
+            if (this.compositeMetricReader == null)
+            {
+                if (state is not List<Metric> metrics)
                 {
                     // TODO: log
                     return;
                 }
 
-            if (metrics.Count == 1)
+                this.reader.RecordDoubleMeasurement(metrics, value, tagsRos);
-            {
-                // special casing the common path
-                // as this is faster than the
-                // foreach, when count is 1.
-                metrics[0].UpdateDouble(value, tagsRos);
             }
             else
             {
-                foreach (var metric in metrics)
+                if (state is not List<List<Metric>> metricsSuperList)
                 {
-                    metric.UpdateDouble(value, tagsRos);
+                    // TODO: log
+                    return;
                 }
+
+                this.compositeMetricReader.RecordDoubleMeasurements(metricsSuperList, value, tagsRos);
             }
         }
 
         internal void MeasurementRecordedLong(Instrument instrument, long value, ReadOnlySpan<KeyValuePair<string, object>> tagsRos, object state)
         {
-            // Get Instrument State
-            var metrics = state as List<Metric>;
-
             Debug.Assert(instrument != null, "instrument must be non-null.");
-            if (metrics == null)
+
+            if (this.compositeMetricReader == null)
+            {
+                if (state is not List<Metric> metrics)
                 {
                     // TODO: log
                     return;
                 }
 
-            if (metrics.Count == 1)
+                this.reader.RecordLongMeasurement(metrics, value, tagsRos);
-            {
-                // special casing the common path
-                // as this is faster than the
-                // foreach, when count is 1.
-                metrics[0].UpdateLong(value, tagsRos);
             }
             else
             {
-                foreach (var metric in metrics)
+                if (state is not List<List<Metric>> metricsSuperList)
                 {
-                    metric.UpdateLong(value, tagsRos);
+                    // TODO: log
+                    return;
                 }
+
+                this.compositeMetricReader.RecordLongMeasurements(metricsSuperList, value, tagsRos);
             }
         }
 
         internal void MeasurementRecordedLongSingleStream(Instrument instrument, long value, ReadOnlySpan<KeyValuePair<string, object>> tagsRos, object state)
         {
-            // Get Instrument State
-            var metric = state as Metric;
-
             Debug.Assert(instrument != null, "instrument must be non-null.");
-            if (metric == null)
+
+            if (this.compositeMetricReader == null)
+            {
+                if (state is not Metric metric)
                 {
                     // TODO: log
                     return;
                 }
 
-            metric.UpdateLong(value, tagsRos);
+                this.reader.RecordSingleStreamLongMeasurement(metric, value, tagsRos);
+            }
+            else
+            {
+                if (state is not List<Metric> metrics)
+                {
+                    // TODO: log
+                    return;
+                }
+
+                this.compositeMetricReader.RecordSingleStreamLongMeasurements(metrics, value, tagsRos);
+            }
         }
 
         internal void MeasurementRecordedDoubleSingleStream(Instrument instrument, double value, ReadOnlySpan<KeyValuePair<string, object>> tagsRos, object state)
         {
-            // Get Instrument State
-            var metric = state as Metric;
-
             Debug.Assert(instrument != null, "instrument must be non-null.");
-            if (metric == null)
+
+            if (this.compositeMetricReader == null)
+            {
+                if (state is not Metric metric)
                 {
                     // TODO: log
                     return;
                 }
 
-            metric.UpdateDouble(value, tagsRos);
+                this.reader.RecordSingleStreamDoubleMeasurement(metric, value, tagsRos);
+            }
+            else
+            {
+                if (state is not List<Metric> metrics)
+                {
+                    // TODO: log
+                    return;
                 }
 
-        internal Batch<Metric> Collect()
+                this.compositeMetricReader.RecordSingleStreamDoubleMeasurements(metrics, value, tagsRos);
+            }
+        }
+
+        internal void CollectObservableInstruments()
         {
             lock (this.collectLock)
-            {
-                try
             {
                 // Record all observable instruments
                 try
@@ -440,40 +415,6 @@ namespace OpenTelemetry.Metrics
                     // threw.
                     OpenTelemetrySdkEventSource.Log.MetricObserverCallbackException(exception);
                 }
-
-                    var indexSnapshot = Math.Min(this.metricIndex, MaxMetrics - 1);
-                    var target = indexSnapshot + 1;
-                    int metricCountCurrentBatch = 0;
-                    for (int i = 0; i < target; i++)
-                    {
-                        var metric = this.metrics[i];
-                        int metricPointSize = 0;
-                        if (metric != null)
-                        {
-                            if (metric.InstrumentDisposed)
-                            {
-                                metricPointSize = metric.Snapshot();
-                                this.metrics[i] = null;
-                            }
-                            else
-                            {
-                                metricPointSize = metric.Snapshot();
-                            }
-
-                            if (metricPointSize > 0)
-                            {
-                                this.metricsCurrentBatch[metricCountCurrentBatch++] = metric;
-                            }
-                        }
-                    }
-
-                    return (metricCountCurrentBatch > 0) ? new Batch<Metric>(this.metricsCurrentBatch, metricCountCurrentBatch) : default;
-                }
-                catch (Exception)
-                {
-                    // TODO: Log
-                    return default;
-                }
             }
         }
 
@@ -538,6 +479,7 @@ namespace OpenTelemetry.Metrics
                     // Wait for up to 5 seconds grace period
                     this.reader?.Shutdown(5000);
                     this.reader?.Dispose();
+                    this.compositeMetricReader?.Dispose();
 
                     this.listener.Dispose();
                 }

src/OpenTelemetry/Metrics/MetricReader.cs

@@ -22,7 +22,10 @@ using OpenTelemetry.Internal;
 
 namespace OpenTelemetry.Metrics
 {
-    public abstract class MetricReader : IDisposable
+    /// <summary>
+    /// MetricReader which does not deal with individual metrics.
+    /// </summary>
+    public abstract partial class MetricReader : IDisposable
     {
         private const AggregationTemporality CumulativeAndDelta = AggregationTemporality.Cumulative | AggregationTemporality.Delta;
         private readonly object newTaskLock = new object();
@@ -211,8 +214,10 @@ namespace OpenTelemetry.Metrics
                 ? null
                 : Stopwatch.StartNew();
 
-            var collectMetric = this.ParentProvider.GetMetricCollect();
+            var collectObservableInstruments = this.ParentProvider.GetObservableInstrumentCollectCallback();
-            var metrics = collectMetric();
+            collectObservableInstruments();
+
+            var metrics = this.GetMetricsBatch();
 
             if (sw == null)
             {

src/OpenTelemetry/Metrics/MetricReaderExt.cs

@@ -0,0 +1,233 @@
+// <copyright file="MetricReaderExt.cs" company="OpenTelemetry Authors">
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+// </copyright>
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.Metrics;
+using OpenTelemetry.Internal;
+
+namespace OpenTelemetry.Metrics
+{
+    /// <summary>
+    /// MetricReader which processes individual metrics.
+    /// </summary>
+    public abstract partial class MetricReader
+    {
+        internal const int MaxMetrics = 1000;
+        private readonly Metric[] metrics = new Metric[MaxMetrics];
+        private readonly Metric[] metricsCurrentBatch = new Metric[MaxMetrics];
+        private readonly HashSet<string> metricStreamNames = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+        private readonly object instrumentCreationLock = new object();
+        private int metricIndex = -1;
+
+        internal Metric AddMetricWithNoViews(Instrument instrument)
+        {
+            var meterName = instrument.Meter.Name;
+            var metricName = instrument.Name;
+            var metricStreamName = $"{meterName}.{metricName}";
+            lock (this.instrumentCreationLock)
+            {
+                if (this.metricStreamNames.Contains(metricStreamName))
+                {
+                    OpenTelemetrySdkEventSource.Log.MetricInstrumentIgnored(metricName, instrument.Meter.Name, "Metric name conflicting with existing name.", "Either change the name of the instrument or change name using View.");
+                    return null;
+                }
+
+                var index = ++this.metricIndex;
+                if (index >= MaxMetrics)
+                {
+                    OpenTelemetrySdkEventSource.Log.MetricInstrumentIgnored(metricName, instrument.Meter.Name, "Maximum allowed Metrics for the provider exceeded.", "Use views to drop unused instruments. Or configure Provider to allow higher limit.");
+                    return null;
+                }
+                else
+                {
+                    var metric = new Metric(instrument, this.preferredAggregationTemporality, metricName, instrument.Description);
+                    this.metrics[index] = metric;
+                    this.metricStreamNames.Add(metricStreamName);
+                    return metric;
+                }
+            }
+        }
+
+        internal void RecordSingleStreamLongMeasurement(Metric metric, long value, ReadOnlySpan<KeyValuePair<string, object>> tags)
+        {
+            metric.UpdateLong(value, tags);
+        }
+
+        internal void RecordSingleStreamDoubleMeasurement(Metric metric, double value, ReadOnlySpan<KeyValuePair<string, object>> tags)
+        {
+            metric.UpdateDouble(value, tags);
+        }
+
+        internal List<Metric> AddMetricsListWithViews(Instrument instrument, List<MetricStreamConfiguration> metricStreamConfigs)
+        {
+            var maxCountMetricsToBeCreated = metricStreamConfigs.Count;
+
+            // Create list with initial capacity as the max metric count.
+            // Due to duplicate/max limit, we may not end up using them
+            // all, and that memory is wasted until Meter disposed.
+            // TODO: Revisit to see if we need to do metrics.TrimExcess()
+            var metrics = new List<Metric>(maxCountMetricsToBeCreated);
+            lock (this.instrumentCreationLock)
+            {
+                for (int i = 0; i < maxCountMetricsToBeCreated; i++)
+                {
+                    var metricStreamConfig = metricStreamConfigs[i];
+                    var meterName = instrument.Meter.Name;
+                    var metricName = metricStreamConfig?.Name ?? instrument.Name;
+                    var metricStreamName = $"{meterName}.{metricName}";
+
+                    if (!MeterProviderBuilderSdk.IsValidInstrumentName(metricName))
+                    {
+                        OpenTelemetrySdkEventSource.Log.MetricInstrumentIgnored(
+                            metricName,
+                            instrument.Meter.Name,
+                            "Metric name is invalid.",
+                            "The name must comply with the OpenTelemetry specification.");
+
+                        continue;
+                    }
+
+                    if (this.metricStreamNames.Contains(metricStreamName))
+                    {
+                        // TODO: Log that instrument is ignored
+                        // as the resulting Metric name is conflicting
+                        // with existing name.
+                        continue;
+                    }
+
+                    if (metricStreamConfig?.Aggregation == Aggregation.Drop)
+                    {
+                        // TODO: Log that instrument is ignored
+                        // as user explicitly asked to drop it
+                        // with View.
+                        continue;
+                    }
+
+                    var index = ++this.metricIndex;
+                    if (index >= MaxMetrics)
+                    {
+                        // TODO: Log that instrument is ignored
+                        // as max number of Metrics have reached.
+                    }
+                    else
+                    {
+                        Metric metric;
+                        var metricDescription = metricStreamConfig?.Description ?? instrument.Description;
+                        string[] tagKeysInteresting = metricStreamConfig?.TagKeys;
+                        double[] histogramBucketBounds = (metricStreamConfig is ExplicitBucketHistogramConfiguration histogramConfig
+                            && histogramConfig.Boundaries != null) ? histogramConfig.Boundaries : null;
+                        metric = new Metric(instrument, this.preferredAggregationTemporality, metricName, metricDescription, histogramBucketBounds, tagKeysInteresting);
+
+                        this.metrics[index] = metric;
+                        metrics.Add(metric);
+                        this.metricStreamNames.Add(metricStreamName);
+                    }
+                }
+
+                return metrics;
+            }
+        }
+
+        internal void RecordLongMeasurement(List<Metric> metrics, long value, ReadOnlySpan<KeyValuePair<string, object>> tags)
+        {
+            if (metrics.Count == 1)
+            {
+                // special casing the common path
+                // as this is faster than the
+                // foreach, when count is 1.
+                metrics[0].UpdateLong(value, tags);
+            }
+            else
+            {
+                foreach (var metric in metrics)
+                {
+                    metric.UpdateLong(value, tags);
+                }
+            }
+        }
+
+        internal void RecordDoubleMeasurement(List<Metric> metrics, double value, ReadOnlySpan<KeyValuePair<string, object>> tags)
+        {
+            if (metrics.Count == 1)
+            {
+                // special casing the common path
+                // as this is faster than the
+                // foreach, when count is 1.
+                metrics[0].UpdateDouble(value, tags);
+            }
+            else
+            {
+                foreach (var metric in metrics)
+                {
+                    metric.UpdateDouble(value, tags);
+                }
+            }
+        }
+
+        internal void CompleteSingleStreamMeasurement(Metric metric)
+        {
+            metric.InstrumentDisposed = true;
+        }
+
+        internal void CompleteMeasurement(List<Metric> metrics)
+        {
+            foreach (var metric in metrics)
+            {
+                metric.InstrumentDisposed = true;
+            }
+        }
+
+        private Batch<Metric> GetMetricsBatch()
+        {
+            try
+            {
+                var indexSnapshot = Math.Min(this.metricIndex, MaxMetrics - 1);
+                var target = indexSnapshot + 1;
+                int metricCountCurrentBatch = 0;
+                for (int i = 0; i < target; i++)
+                {
+                    var metric = this.metrics[i];
+                    int metricPointSize = 0;
+                    if (metric != null)
+                    {
+                        if (metric.InstrumentDisposed)
+                        {
+                            metricPointSize = metric.Snapshot();
+                            this.metrics[i] = null;
+                        }
+                        else
+                        {
+                            metricPointSize = metric.Snapshot();
+                        }
+
+                        if (metricPointSize > 0)
+                        {
+                            this.metricsCurrentBatch[metricCountCurrentBatch++] = metric;
+                        }
+                    }
+                }
+
+                return (metricCountCurrentBatch > 0) ? new Batch<Metric>(this.metricsCurrentBatch, metricCountCurrentBatch) : default;
+            }
+            catch (Exception)
+            {
+                // TODO: Log
+                return default;
+            }
+        }
+    }
+}

src/OpenTelemetry/ProviderExtensions.cs

@@ -50,16 +50,6 @@ namespace OpenTelemetry
             return Resource.Empty;
         }
 
-        public static Func<Batch<Metric>> GetMetricCollect(this BaseProvider baseProvider)
-        {
-            if (baseProvider is MeterProviderSdk meterProviderSdk)
-            {
-                return meterProviderSdk.Collect;
-            }
-
-            return null;
-        }
-
         /// <summary>
         /// Gets the <see cref="Resource"/> associated with the <see cref="BaseProvider"/>.
         /// </summary>
@@ -69,5 +59,15 @@ namespace OpenTelemetry
         {
             return ResourceBuilder.CreateDefault().Build();
         }
+
+        internal static Action GetObservableInstrumentCollectCallback(this BaseProvider baseProvider)
+        {
+            if (baseProvider is MeterProviderSdk meterProviderSdk)
+            {
+                return meterProviderSdk.CollectObservableInstruments;
+            }
+
+            return null;
+        }
     }
 }

test/OpenTelemetry.Tests/Metrics/MetricAPITest.cs

@@ -690,6 +690,28 @@ namespace OpenTelemetry.Metrics.Tests
             Assert.Equal(name, metric.Name);
         }
 
+        [Theory]
+        [InlineData(false)]
+        [InlineData(true)]
+        public void SetupSdkProviderWithNoReader(bool hasViews)
+        {
+            // This test ensures that MeterProviderSdk can be set up without any reader
+            using var meter = new Meter($"{Utils.GetCurrentMethodName()}.{hasViews}");
+            var meterProviderBuilder = Sdk.CreateMeterProviderBuilder()
+                .AddMeter(meter.Name);
+
+            if (hasViews)
+            {
+                meterProviderBuilder.AddView("counter", "renamedCounter");
+            }
+
+            using var meterProvider = meterProviderBuilder.Build();
+
+            var counter = meter.CreateCounter<long>("counter");
+
+            counter.Add(10, new KeyValuePair<string, object>("key", "value"));
+        }
+
         private static long GetLongSum(List<Metric> metrics)
         {
             long sum = 0;

test/OpenTelemetry.Tests/Metrics/MultipleReadersTests.cs

@@ -0,0 +1,119 @@
+// <copyright file="MultipleReadersTests.cs" company="OpenTelemetry Authors">
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+// </copyright>
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.Metrics;
+using OpenTelemetry.Exporter;
+using OpenTelemetry.Tests;
+using Xunit;
+
+namespace OpenTelemetry.Metrics.Tests
+{
+    public class MultipleReadersTests
+    {
+        [Theory]
+        [InlineData(AggregationTemporality.Delta, false)]
+        [InlineData(AggregationTemporality.Delta, true)]
+        [InlineData(AggregationTemporality.Cumulative, false)]
+        [InlineData(AggregationTemporality.Cumulative, true)]
+        public void SdkSupportsMultipleReaders(AggregationTemporality aggregationTemporality, bool hasViews)
+        {
+            var exporterdMetricItems1 = new List<Metric>();
+            using var deltaMetricExporter1 = new InMemoryExporter<Metric>(exporterdMetricItems1);
+            using var deltaMetricReader1 = new BaseExportingMetricReader(deltaMetricExporter1)
+            {
+                PreferredAggregationTemporality = AggregationTemporality.Delta,
+            };
+
+            var exporterdMetricItems2 = new List<Metric>();
+            using var deltaMetricExporter2 = new InMemoryExporter<Metric>(exporterdMetricItems2);
+            using var deltaMetricReader2 = new BaseExportingMetricReader(deltaMetricExporter2)
+            {
+                PreferredAggregationTemporality = aggregationTemporality,
+            };
+            using var meter = new Meter($"{Utils.GetCurrentMethodName()}.{aggregationTemporality}.{hasViews}");
+
+            var counter = meter.CreateCounter<long>("counter");
+
+            int index = 0;
+            var values = new long[] { 100, 200, 300, 400 };
+            long GetValue() => values[index++];
+            var gauge = meter.CreateObservableGauge("gauge", () => GetValue());
+
+            var meterProviderBuilder = Sdk.CreateMeterProviderBuilder()
+                .AddMeter(meter.Name)
+                .AddReader(deltaMetricReader1)
+                .AddReader(deltaMetricReader2);
+
+            if (hasViews)
+            {
+                meterProviderBuilder.AddView("counter", "renamedCounter");
+            }
+
+            using var meterProvider = meterProviderBuilder.Build();
+
+            counter.Add(10, new KeyValuePair<string, object>("key", "value"));
+
+            meterProvider.ForceFlush();
+
+            Assert.Equal(2, exporterdMetricItems1.Count);
+            Assert.Equal(2, exporterdMetricItems2.Count);
+
+            // Check value exported for Counter
+            this.AssertLongSumValueForMetric(exporterdMetricItems1[0], 10);
+            this.AssertLongSumValueForMetric(exporterdMetricItems2[0], 10);
+
+            // Check value exported for Gauge
+            this.AssertLongSumValueForMetric(exporterdMetricItems1[1], 100);
+            this.AssertLongSumValueForMetric(exporterdMetricItems2[1], 200);
+
+            exporterdMetricItems1.Clear();
+            exporterdMetricItems2.Clear();
+
+            counter.Add(15, new KeyValuePair<string, object>("key", "value"));
+
+            meterProvider.ForceFlush();
+
+            Assert.Equal(2, exporterdMetricItems1.Count);
+            Assert.Equal(2, exporterdMetricItems2.Count);
+
+            // Check value exported for Counter
+            this.AssertLongSumValueForMetric(exporterdMetricItems1[0], 15);
+            if (aggregationTemporality == AggregationTemporality.Delta)
+            {
+                this.AssertLongSumValueForMetric(exporterdMetricItems2[0], 15);
+            }
+            else
+            {
+                this.AssertLongSumValueForMetric(exporterdMetricItems2[0], 25);
+            }
+
+            // Check value exported for Gauge
+            this.AssertLongSumValueForMetric(exporterdMetricItems1[1], 300);
+            this.AssertLongSumValueForMetric(exporterdMetricItems2[1], 400);
+        }
+
+        private void AssertLongSumValueForMetric(Metric metric, long value)
+        {
+            using var metricPoints = metric.GetMetricPoints();
+            var metricPointsEnumerator = metricPoints.GetEnumerator();
+            Assert.True(metricPointsEnumerator.MoveNext()); // One MetricPoint is emitted for the Metric
+            ref var metricPointForFirstExport = ref metricPointsEnumerator.Current;
+            Assert.Equal(value, metricPointForFirstExport.LongValue);
+        }
+    }
+}