add Copy methods to MetricPoint and HistogramBuckets to facilitate deep-copying (#3113)

src/OpenTelemetry/Metrics/HistogramBuckets.cs

@@ -14,6 +14,8 @@
 // limitations under the License.
 // </copyright>
 
+using System;
+
 namespace OpenTelemetry.Metrics
 {
     /// <summary>
@@ -45,6 +47,16 @@ namespace OpenTelemetry.Metrics
 
         public Enumerator GetEnumerator() => new(this);
 
+        internal HistogramBuckets Copy()
+        {
+            HistogramBuckets copy = new HistogramBuckets(this.ExplicitBounds);
+
+            Array.Copy(this.SnapshotBucketCounts, copy.SnapshotBucketCounts, this.SnapshotBucketCounts.Length);
+            copy.SnapshotSum = this.SnapshotSum;
+
+            return copy;
+        }
+
         /// <summary>
         /// Enumerates the elements of a <see cref="HistogramBuckets"/>.
         /// </summary>

src/OpenTelemetry/Metrics/MetricPoint.cs

@@ -30,7 +30,7 @@ namespace OpenTelemetry.Metrics
 
         private readonly AggregationType aggType;
 
-        private readonly HistogramBuckets histogramBuckets;
+        private HistogramBuckets histogramBuckets;
 
         // Represents temporality adjusted "value" for double/long metric types or "count" when histogram
         private MetricPointValueStorage runningValue;
@@ -231,6 +231,13 @@ namespace OpenTelemetry.Metrics
             return this.histogramBuckets;
         }
 
+        internal readonly MetricPoint Copy()
+        {
+            MetricPoint copy = this;
+            copy.histogramBuckets = this.histogramBuckets?.Copy();
+            return copy;
+        }
+
         internal void Update(long number)
         {
             switch (this.aggType)

test/OpenTelemetry.Tests/Metrics/MetricPointTests.cs

@@ -0,0 +1,104 @@
+// <copyright file="MetricPointTests.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;
+using System.Threading;
+using OpenTelemetry.Tests;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace OpenTelemetry.Metrics.Tests
+{
+    public class MetricPointTests : IDisposable
+    {
+        private Meter meter;
+        private MeterProvider provider;
+        private Metric metric;
+        private MetricPoint metricPoint;
+
+        private Histogram<long> histogram;
+        private double[] bounds;
+
+        public MetricPointTests()
+        {
+            this.meter = new Meter(Utils.GetCurrentMethodName());
+            this.histogram = this.meter.CreateHistogram<long>("histogram");
+
+            // Evenly distribute the bound values over the range [0, MaxValue)
+            this.bounds = new double[10];
+            for (int i = 0; i < this.bounds.Length; i++)
+            {
+                this.bounds[i] = i * 1000 / this.bounds.Length;
+            }
+
+            var exportedItems = new List<Metric>();
+
+            this.provider = Sdk.CreateMeterProviderBuilder()
+                .AddMeter(this.meter.Name)
+                .AddInMemoryExporter(exportedItems)
+                .AddView(this.histogram.Name, new ExplicitBucketHistogramConfiguration() { Boundaries = this.bounds })
+                .Build();
+
+            this.histogram.Record(500);
+
+            this.provider.ForceFlush();
+
+            this.metric = exportedItems[0];
+            var metricPointsEnumerator = this.metric.GetMetricPoints().GetEnumerator();
+            metricPointsEnumerator.MoveNext();
+            this.metricPoint = metricPointsEnumerator.Current;
+        }
+
+        public void Dispose()
+        {
+            this.meter?.Dispose();
+            this.provider?.Dispose();
+        }
+
+        [Fact]
+        public void VerifyMetricPointCopy()
+        {
+            var copy = this.metricPoint.Copy();
+
+            // Verify these structs are unique instances.
+            Assert.NotEqual(copy, this.metricPoint);
+
+            // Verify properties are copied.
+            Assert.Equal(copy.Tags, this.metricPoint.Tags);
+            Assert.Equal(copy.StartTime, this.metricPoint.StartTime);
+            Assert.Equal(copy.EndTime, this.metricPoint.EndTime);
+        }
+
+        [Fact]
+        public void VerifyHistogramBucketsCopy()
+        {
+            var histogramBuckets = this.metricPoint.GetHistogramBuckets();
+            var copy = histogramBuckets.Copy();
+
+            // Verify these are unique instances.
+            Assert.False(ReferenceEquals(copy, histogramBuckets));
+            Assert.NotSame(copy, histogramBuckets);
+
+            // Verify fields are copied
+            Assert.NotSame(copy.SnapshotBucketCounts, histogramBuckets.SnapshotBucketCounts);
+            Assert.Equal(copy.SnapshotBucketCounts, histogramBuckets.SnapshotBucketCounts);
+            Assert.Equal(copy.SnapshotSum, histogramBuckets.SnapshotSum);
+        }
+    }
+}