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Customizing OpenTelemetry .NET SDK for Metrics

MeterProvider

As shown in the getting-started doc, a valid MeterProvider must be configured and built to collect metrics with OpenTelemetry .NET Sdk. MeterProvider holds all the configuration for metrics like MetricReaders, Views, etc. Naturally, almost all the customizations must be done on the MeterProvider.

Building a MeterProvider

Building a MeterProvider is done using MeterProviderBuilder which must be obtained by calling Sdk.CreateMeterProviderBuilder(). MeterProviderBuilder exposes various methods which configure the provider it is going to build. These include methods like AddMeter, AddView etc, and are explained in subsequent sections of this document. Once configuration is done, calling Build() on the MeterProviderBuilder builds the MeterProvider instance. Once built, changes to its configuration is not allowed. In most cases, a single MeterProvider is created at the application startup, and is disposed when application shuts down.

The snippet below shows how to build a basic MeterProvider. This will create a provider with default configuration, and is not particularly useful. The subsequent sections show how to build a more useful provider.

using OpenTelemetry;
using OpenTelemetry.Metrics;

using var meterProvider = Sdk.CreateMeterProviderBuilder().Build();

In a typical application, a single MeterProvider is created at application startup and disposed at application shutdown. It is important to ensure that the provider is not disposed too early. Actual mechanism depends on the application type. For example, in a typical ASP.NET application, MeterProvider is created in Application_Start, and disposed in Application_End (both methods are a part of the Global.asax.cs file) as shown here. In a typical ASP.NET Core application, MeterProvider lifetime is managed by leveraging the built-in Dependency Injection container as shown here.

MeterProvider configuration

MeterProvider holds the metrics configuration, which includes the following:

  1. The list of Meters from which instruments are created to report measurements.
  2. The list of instrumentations enabled via Instrumentation Library.
  3. The list of MetricReaders, including exporting readers which exports metrics to Exporters
  4. The Resource associated with the metrics.
  5. The list of Views to be used.

Meter

Meter is used for creating Instruments, which are then used to report Measurements. The SDK follows an explicit opt-in model for listening to meters. i.e, by default, it listens to no meters. Every meter which is used to create instruments must be explicitly added to the meter provider.

AddMeter method on MeterProviderBuilder can be used to add a Meter to the provider. The name of the Meter (case-insensitive) must be provided as an argument to this method. AddMeter can be called multiple times to add more than one meters. It also supports wildcard subscription model. It is important to note that all the instruments from the meter will be enabled, when a Meter is added. To selectively drop some instruments from a Meter, use the View feature, as shown here.

It is not possible to add meters once the provider is built by the Build() method on the MeterProviderBuilder.

The snippet below shows how to add meters to the provider.

using OpenTelemetry;
using OpenTelemetry.Metrics;

using var meterProvider = Sdk.CreateMeterProviderBuilder()
    // The following enables instruments from Meter
    // named "MyCompany.MyProduct.MyLibrary" only.
    .AddMeter("MyCompany.MyProduct.MyLibrary")
    // The following enables instruments from all Meters
    // whose name starts with  "AbcCompany.XyzProduct.".
    .AddMeter("AbcCompany.XyzProduct.*")
    .Build();

See Program.cs for complete example.

Note: A common mistake while configuring MeterProvider is forgetting to add the required Meters to the provider. It is recommended to leverage the wildcard subscription model where it makes sense. For example, if your application is expecting to enable instruments from a number of libraries from a company "Abc", the you can use AddMeter("Abc.*") to enable all meters whose name starts with "Abc.".

View

A View provides the ability to customize the metrics that are output by the SDK. Following sections explains how to use this feature. Each section has two code snippets. The first one uses an overload of AddView method that takes in the name of the instrument as the first parameter. The View configuration is then applied to the matching instrument name. The second code snippet shows how to use an advanced selection criteria to achieve the same results. This requires the user to provide a Func<Instrument, MetricStreamConfiguration> which offers more flexibility in filtering the instruments to which the View should be applied.

Rename an instrument

When SDK produces Metrics, the name of Metric is by default the name of the instrument. View may be used to rename a metric to a different name. This is particularly useful if there are conflicting instrument names, and you do not own the instrument to create it with a different name.

    // Rename an instrument to new name.
    .AddView(instrumentName: "MyCounter", name: "MyCounterRenamed")

    // Advanced selection criteria and config via Func<Instrument, MetricStreamConfiguration>
    .AddView((instrument) =>
    {
        if (instrument.Meter.Name == "CompanyA.ProductB.LibraryC" &&
            instrument.Name == "MyCounter")
        {
            return new MetricStreamConfiguration() { Name = "MyCounterRenamed" };
        }

        return null;
    })

Drop an instrument

When using AddMeter to add a Meter to the provider, all the instruments from that Meter gets subscribed. Views can be used to selectively drop an instrument from a Meter. If the goal is to drop every instrument from a Meter, then it is recommended to simply not add that Meter using AddMeter.

    // Drop the instrument "MyCounterDrop".
    .AddView(instrumentName: "MyCounterDrop", MetricStreamConfiguration.Drop)

    // Advanced selection criteria and config via Func<Instrument, MetricStreamConfiguration>
    .AddView((instrument) =>
    {
        if (instrument.Meter.Name == "CompanyA.ProductB.LibraryC" &&
            instrument.Name == "MyCounterDrop")
        {
            return MetricStreamConfiguration.Drop;
        }

        return null;
    })

Select specific tags

When recording a measurement from an instrument, all the tags that were provided are reported as dimensions for the given metric. Views can be used to selectively choose a subset of dimensions to report for a given metric. This is useful when you have a metric for which only a few of the dimensions associated with the metric are of interest to you.

    // Only choose "name" as the dimension for the metric "MyFruitCounter"
    .AddView(
        instrumentName: "MyFruitCounter",
        metricStreamConfiguration: new MetricStreamConfiguration
        {
            TagKeys = new string[] { "name" },
        })

    ...
    // Only the dimension "name" is selected, "color" is dropped
    MyFruitCounter.Add(1, new("name", "apple"), new("color", "red"));
    MyFruitCounter.Add(2, new("name", "lemon"), new("color", "yellow"));
    MyFruitCounter.Add(2, new("name", "apple"), new("color", "green"));
    ...

    // If you provide an empty `string` array as `TagKeys` to the `MetricStreamConfiguration`
    // the SDK will drop all the dimensions associated with the metric
    .AddView(
        instrumentName: "MyFruitCounter",
        metricStreamConfiguration: new MetricStreamConfiguration
        {
            TagKeys = new string[] { },
        })

    ...
    // both "name" and "color" are dropped
    MyFruitCounter.Add(1, new("name", "apple"), new("color", "red"));
    MyFruitCounter.Add(2, new("name", "lemon"), new("color", "yellow"));
    MyFruitCounter.Add(2, new("name", "apple"), new("color", "green"));
    ...

    // Advanced selection criteria and config via Func<Instrument, MetricStreamConfiguration>
    .AddView((instrument) =>
    {
        if (instrument.Meter.Name == "CompanyA.ProductB.LibraryC" &&
            instrument.Name == "MyFruitCounter")
        {
            return new MetricStreamConfiguration
            {
                TagKeys = new string[] { "name" },
            };
        }

        return null;
    })

Specify custom boundaries for Histogram

By default, the boundaries used for a Histogram are { 0, 5, 10, 25, 50, 75, 100, 250, 500, 1000}. Views can be used to provide custom boundaries for a Histogram. The measurements are then aggregated using the custom boundaries provided instead of the the default boundaries. This requires the use of ExplicitBucketHistogramConfiguration.

    // Change Histogram boundaries to count measurements under the following buckets:
    // (-inf, 10]
    // (10, 20]
    // (20, +inf)
    .AddView(
        instrumentName: "MyHistogram",
        new ExplicitBucketHistogramConfiguration { Boundaries = new double[] { 10, 20 } })

    // If you provide an empty `double` array as `Boundaries` to the `ExplicitBucketHistogramConfiguration`,
    // the SDK will only export the sum and count for the measurements.
    // There are no buckets exported in this case.
    .AddView(
        instrumentName: "MyHistogram",
        new ExplicitBucketHistogramConfiguration { Boundaries = new double[] { } })

    // Advanced selection criteria and config via Func<Instrument, MetricStreamConfiguration>
    .AddView((instrument) =>
    {
        if (instrument.Meter.Name == "CompanyA.ProductB.LibraryC" &&
            instrument.Name == "MyHistogram")
        {
            // `ExplicitBucketHistogramConfiguration` is a child class of `MetricStreamConfiguration`
            return new ExplicitBucketHistogramConfiguration
            {
                Boundaries = new double[] { 10, 20 },
            };
        }

        return null;
    })

NOTE: The SDK currently does not support any changes to Aggregation type by using Views.

See Program.cs for a complete example.

Changing maximum Metric Streams

Every instrument results in the creation of a single Metric stream. With Views, it is possible to produce more than one Metric stream from a single instrument. To protect the SDK from unbounded memory usage, SDK limits the maximum number of metric streams. All the measurements from the instruments created after reaching this limit will be dropped. The default is 1000, and SetMaxMetricStreams can be used to override the default.

Consider the below example. Here we set the maximum number of MetricStreams allowed to be 1. This means that the SDK would export measurements from only one MetricStream. The very first instrument that is published (MyFruitCounter in this case) will create a MetricStream and the SDK will thereby reach the maximum MetricStream limit of 1. The measurements from any subsequent instruments added will be dropped.

using System.Diagnostics.Metrics;
using OpenTelemetry;
using OpenTelemetry.Metrics;

Counter<long> MyFruitCounter = MyMeter.CreateCounter<long>("MyFruitCounter");
Counter<long> AnotherFruitCounter = MyMeter.CreateCounter<long>("AnotherFruitCounter");

using var meterProvider = Sdk.CreateMeterProviderBuilder()
    .AddMeter("*")
    .AddConsoleExporter()
    .SetMaxMetricStreams(1) // The default value is 1000
    .Build();

// SDK only exports measurements from `MyFruitCounter`.
MyFruitCounter.Add(1, new("name", "apple"), new("color", "red"));

// The measurements from `AnotherFruitCounter` are dropped as the maximum
// `MetricStream`s allowed is `1`.
AnotherFruitCounter.Add(1, new("name", "apple"), new("color", "red"));

Changing maximum MetricPoints per MetricStream

A Metric stream can contain as many Metric points as the number of unique combination of keys and values. To protect the SDK from unbounded memory usage, SDK limits the maximum number of metric points per metric stream, to a default of 2000. Once the limit is hit, any new key/value combination for that metric is ignored. The SDK chooses the key/value combinations in the order in which they are emitted. SetMaxMetricPointsPerMetricStream can be used to override the default.

NOTE: One MetricPoint is reserved for every MetricStream for the special case where there is no key/value pair associated with the metric. The maximum number of MetricPoints has to accommodate for this special case.

Consider the below example. Here we set the maximum number of MetricPoints allowed to be 3. This means that for every MetricStream, the SDK will export measurements for up to 3 distinct key/value combinations of the metric. There are two instruments published here: MyFruitCounter and AnotherFruitCounter. There are two total MetricStreams created one for each of these instruments. SDK will limit the maximum number of distinct key/value combinations for each of these MetricStreams to 3.

using System.Collections.Generic;
using System.Diagnostics.Metrics;
using OpenTelemetry;
using OpenTelemetry.Metrics;

Counter<long> MyFruitCounter = MyMeter.CreateCounter<long>("MyFruitCounter");
Counter<long> AnotherFruitCounter = MyMeter.CreateCounter<long>("AnotherFruitCounter");

using var meterProvider = Sdk.CreateMeterProviderBuilder()
    .AddMeter("*")
    .AddConsoleExporter()
    .SetMaxMetricPointsPerMetricStream(3) // The default value is 2000
    .Build();

// There are four distinct key/value combinations emitted for `MyFruitCounter`:
// 1. No key/value pair
// 2. (name:apple, color:red)
// 3. (name:lemon, color:yellow)
// 4. (name:apple, color:green)

// Since the maximum number of `MetricPoint`s allowed is `3`, the SDK will only export measurements for the following three combinations:
// 1. No key/value pair
// 2. (name:apple, color:red)
// 3. (name:lemon, color:yellow)

MyFruitCounter.Add(1); // Exported (No key/value pair)
MyFruitCounter.Add(1, new("name", "apple"), new("color", "red")); // Exported
MyFruitCounter.Add(2, new("name", "lemon"), new("color", "yellow")); // Exported
MyFruitCounter.Add(1, new("name", "lemon"), new("color", "yellow")); // Exported
MyFruitCounter.Add(2, new("name", "apple"), new("color", "green")); // Not exported
MyFruitCounter.Add(5, new("name", "apple"), new("color", "red")); // Exported
MyFruitCounter.Add(4, new("name", "lemon"), new("color", "yellow")); // Exported

// There are four distinct key/value combinations emitted for `AnotherFruitCounter`:
// 1. (name:kiwi)
// 2. (name:banana, color:yellow)
// 3. (name:mango, color:yellow)
// 4. (name:banana, color:green)

// Since the maximum number of `MetricPoint`s allowed is `3`, the SDK will only export measurements for the following three combinations:
// 1. No key/value pair (This is a special case. The SDK reserves a `MetricPoint` for it even if it's not explicitly emitted.)
// 2. (name:kiwi)
// 3. (name:banana, color:yellow)

AnotherFruitCounter.Add(4, new KeyValuePair<string, object>("name", "kiwi")); // Exported
AnotherFruitCounter.Add(1, new("name", "banana"), new("color", "yellow")); // Exported
AnotherFruitCounter.Add(2, new("name", "mango"), new("color", "yellow")); // Not exported
AnotherFruitCounter.Add(1, new("name", "mango"), new("color", "yellow")); // Not exported
AnotherFruitCounter.Add(2, new("name", "banana"), new("color", "green")); // Not exported
AnotherFruitCounter.Add(5, new("name", "banana"), new("color", "yellow")); // Exported
AnotherFruitCounter.Add(4, new("name", "mango"), new("color", "yellow")); // Not exported

NOTE: The above limit is per metric stream, and applies to all the metric streams. There is no ability to apply different limits for each instrument at this moment.

Instrumentation

// TODO

MetricReader

MetricReader allows collecting the pre-aggregated metrics from the SDK. They are typically paired with a MetricExporter which does the actual export of metrics.

Though MetricReader can be added by using the AddReader method on MeterProviderBuilder, most users use the extension methods on MeterProviderBuilder offered by exporter libraries, which adds the correct MetricReader, that is configured to export metrics to the exporter.

Refer to the individual exporter docs to learn how to use them:

Resource

// TODO