Add support for OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/CHANGELOG.md

@@ -26,6 +26,13 @@ Notes](../../RELEASENOTES.md).
 * Fix `NullReferenceException` when no bucket boundaries configured for a view.
   ([#6773](https://github.com/open-telemetry/opentelemetry-dotnet/pull/6773))
 
+* Added support for `OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION`
+  environment variable to configure the default histogram aggregation for
+  histogram instruments. Valid values are `explicit_bucket_histogram` (default)
+  and `base2_exponential_bucket_histogram`. Explicit views configured via
+  `AddView` take precedence over this setting.
+  ([#6778](https://github.com/open-telemetry/opentelemetry-dotnet/pull/6778))
+
 ## 1.14.0
 
 Released 2025-Nov-12

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Implementation/OtlpServiceCollectionExtensions.cs

@@ -34,6 +34,21 @@ public static void AddOtlpExporterMetricsServices(this IServiceCollection servic
                 {
                     readerOptions.TemporalityPreference = enumValue;
                 }
+
+                // Parse histogram aggregation using direct string comparison instead of Enum.TryParse.
+                // The spec defines snake_case values (explicit_bucket_histogram, base2_exponential_bucket_histogram).
+                // Using direct string comparison ensures we strictly validate against spec-defined values and fail
+                // gracefully for invalid inputs, rather than attempting to parse arbitrary strings to enum values.
+                // Case-insensitive comparison is used for flexibility, though the spec uses lowercase.
+                var otlpDefaultHistogramAggregation = config[OtlpSpecConfigDefinitions.MetricsDefaultHistogramAggregationEnvVarName];
+                if (string.Equals(otlpDefaultHistogramAggregation, "base2_exponential_bucket_histogram", StringComparison.OrdinalIgnoreCase))
+                {
+                    readerOptions.DefaultHistogramAggregation = MetricReaderHistogramAggregation.Base2ExponentialBucketHistogram;
+                }
+                else if (string.Equals(otlpDefaultHistogramAggregation, "explicit_bucket_histogram", StringComparison.OrdinalIgnoreCase))
+                {
+                    readerOptions.DefaultHistogramAggregation = MetricReaderHistogramAggregation.ExplicitBucketHistogram;
+                }
             });
     }
 

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Implementation/OtlpSpecConfigDefinitions.cs

@@ -26,6 +26,7 @@ internal static class OtlpSpecConfigDefinitions
     public const string MetricsTimeoutEnvVarName = "OTEL_EXPORTER_OTLP_METRICS_TIMEOUT";
     public const string MetricsProtocolEnvVarName = "OTEL_EXPORTER_OTLP_METRICS_PROTOCOL";
     public const string MetricsTemporalityPreferenceEnvVarName = "OTEL_EXPORTER_OTLP_METRICS_TEMPORALITY_PREFERENCE";
+    public const string MetricsDefaultHistogramAggregationEnvVarName = "OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION";
 
     public const string TracesEndpointEnvVarName = "OTEL_EXPORTER_OTLP_TRACES_ENDPOINT";
     public const string TracesHeadersEnvVarName = "OTEL_EXPORTER_OTLP_TRACES_HEADERS";

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/README.md

@@ -484,13 +484,21 @@ or reader
 
 * Metrics:
 
-  The following environment variables can be used to override the default value
-  of the `TemporalityPreference` setting for the reader configured for metrics
-  when using OTLP exporter:
+  The following environment variables can be used to override the default values
+  for the reader configured for metrics when using OTLP exporter:
 
-  | Environment variable                                | `MetricReaderOptions` property                  |
-  | ----------------------------------------------------| ------------------------------------------------|
-  | `OTEL_EXPORTER_OTLP_METRICS_TEMPORALITY_PREFERENCE` | `TemporalityPreference`                         |
+  | Environment variable                                        | `MetricReaderOptions` property                  |
+  | ------------------------------------------------------------| ------------------------------------------------|
+  | `OTEL_EXPORTER_OTLP_METRICS_TEMPORALITY_PREFERENCE`         | `TemporalityPreference`                         |
+  | `OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION`  | `DefaultHistogramAggregation`                   |
+
+  `OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION` specifies the
+  default aggregation to use for histogram instruments. Valid values are:
+  * `explicit_bucket_histogram` (default)
+  * `base2_exponential_bucket_histogram`
+
+  Note: Explicit views configured via `AddView` take precedence over the
+  default histogram aggregation.
 
   The following environment variables can be used to override the default values
   of the periodic exporting metric reader configured for metrics:

src/OpenTelemetry/Metrics/Reader/MetricReader.cs

@@ -93,6 +93,11 @@ public MetricReaderTemporalityPreference TemporalityPreference
         }
     }
 
+    /// <summary>
+    /// Gets or sets the default histogram aggregation.
+    /// </summary>
+    internal MetricReaderHistogramAggregation? DefaultHistogramAggregation { get; set; }
+
     /// <summary>
     /// Attempts to collect the metrics, blocks the current thread until
     /// metrics collection completed, shutdown signaled or timed out.

src/OpenTelemetry/Metrics/Reader/MetricReaderExt.cs

@@ -52,7 +52,15 @@ internal virtual List<Metric> AddMetricWithNoViews(Instrument instrument)
         Debug.Assert(instrument != null, "instrument was null");
         Debug.Assert(this.metrics != null, "this.metrics was null");
 
-        var metricStreamIdentity = new MetricStreamIdentity(instrument!, metricStreamConfiguration: null);
+        MetricStreamConfiguration? metricStreamConfiguration = null;
+
+        // Apply default histogram aggregation if configured
+        if (this.DefaultHistogramAggregation is { } aggregation)
+        {
+            metricStreamConfiguration = CreateDefaultHistogramConfiguration(instrument!, aggregation);
+        }
+
+        var metricStreamIdentity = new MetricStreamIdentity(instrument!, metricStreamConfiguration);
 
         var exemplarFilter = metricStreamIdentity.IsHistogram
             ? this.exemplarFilterForHistograms ?? this.exemplarFilter
@@ -120,6 +128,13 @@ internal virtual List<Metric> AddMetricWithViews(Instrument instrument, List<Met
             for (int i = 0; i < maxCountMetricsToBeCreated; i++)
             {
                 var metricStreamConfig = metricStreamConfigs[i];
+
+                // Apply default histogram aggregation if no explicit view is provided
+                if (metricStreamConfig == null && this.DefaultHistogramAggregation is { } aggregation)
+                {
+                    metricStreamConfig = CreateDefaultHistogramConfiguration(instrument!, aggregation);
+                }
+
                 var metricStreamIdentity = new MetricStreamIdentity(instrument!, metricStreamConfig);
 
                 var exemplarFilter = metricStreamIdentity.IsHistogram
@@ -189,6 +204,25 @@ internal void ApplyParentProviderSettings(
         this.exemplarFilterForHistograms = exemplarFilterForHistograms;
     }
 
+    private static MetricStreamConfiguration? CreateDefaultHistogramConfiguration(Instrument instrument, MetricReaderHistogramAggregation aggregation)
+    {
+        Debug.Assert(instrument != null, "instrument was null");
+
+        var instrumentType = instrument!.GetType();
+        if (instrumentType.IsGenericType)
+        {
+            var genericType = instrumentType.GetGenericTypeDefinition();
+            if (genericType == typeof(Histogram<>))
+            {
+                return aggregation == MetricReaderHistogramAggregation.Base2ExponentialBucketHistogram
+                    ? new Base2ExponentialBucketHistogramConfiguration()
+                    : new ExplicitBucketHistogramConfiguration();
+            }
+        }
+
+        return null;
+    }
+
     private bool TryGetExistingMetric(in MetricStreamIdentity metricStreamIdentity, [NotNullWhen(true)] out Metric? existingMetric)
         => this.instrumentIdentityToMetric.TryGetValue(metricStreamIdentity, out existingMetric)
             && existingMetric != null && existingMetric.Active;

src/OpenTelemetry/Metrics/Reader/MetricReaderHistogramAggregation.cs

@@ -0,0 +1,20 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+namespace OpenTelemetry.Metrics;
+
+/// <summary>
+/// Defines the default histogram aggregation for a <see cref="MetricReader" />.
+/// </summary>
+internal enum MetricReaderHistogramAggregation
+{
+    /// <summary>
+    /// Explicit bucket histogram aggregation.
+    /// </summary>
+    ExplicitBucketHistogram = 0,
+
+    /// <summary>
+    /// Base2 exponential bucket histogram aggregation.
+    /// </summary>
+    Base2ExponentialBucketHistogram = 1,
+}

src/OpenTelemetry/Metrics/Reader/MetricReaderOptions.cs

@@ -48,4 +48,9 @@ public PeriodicExportingMetricReaderOptions PeriodicExportingMetricReaderOptions
             this.periodicExportingMetricReaderOptions = value;
         }
     }
+
+    /// <summary>
+    /// Gets or sets the default histogram aggregation.
+    /// </summary>
+    internal MetricReaderHistogramAggregation? DefaultHistogramAggregation { get; set; }
 }

src/Shared/PeriodicExportingMetricReaderHelper.cs

@@ -23,6 +23,7 @@ internal static PeriodicExportingMetricReader CreatePeriodicExportingMetricReade
         var metricReader = new PeriodicExportingMetricReader(exporter, exportInterval, exportTimeout)
         {
             TemporalityPreference = options.TemporalityPreference,
+            DefaultHistogramAggregation = options.DefaultHistogramAggregation,
         };
 
         return metricReader;

test/OpenTelemetry.Exporter.OpenTelemetryProtocol.Tests/OpenTelemetry.Exporter.OpenTelemetryProtocol.Tests.csproj

@@ -44,6 +44,7 @@
   </ItemGroup>
 
   <ItemGroup>
+    <Compile Include="$(RepoRoot)\test\OpenTelemetry.Tests\EnvironmentVariableScope.cs" Link="Includes\EnvironmentVariableScope.cs" />
     <Compile Include="$(RepoRoot)\test\OpenTelemetry.Tests\Shared\DelegatingExporter.cs" Link="Includes\DelegatingExporter.cs" />
     <Compile Include="$(RepoRoot)\test\OpenTelemetry.Tests\Shared\EventSourceTestHelper.cs" Link="Includes\EventSourceTestHelper.cs" />
     <Compile Include="$(RepoRoot)\test\OpenTelemetry.Tests\Shared\SkipUnlessEnvVarFoundTheoryAttribute.cs" Link="Includes\SkipUnlessEnvVarFoundTheoryAttribute.cs" />

test/OpenTelemetry.Exporter.OpenTelemetryProtocol.Tests/OtlpMetricsExporterTests.cs

@@ -971,6 +971,162 @@ public void Dispose()
         GC.SuppressFinalize(this);
     }
 
+    [Theory]
+    [InlineData("explicit_bucket_histogram", MetricReaderHistogramAggregation.ExplicitBucketHistogram)]
+    [InlineData("base2_exponential_bucket_histogram", MetricReaderHistogramAggregation.Base2ExponentialBucketHistogram)]
+    [InlineData("invalid", null)]
+    internal void TestDefaultHistogramAggregationUsingConfiguration(string configValue, MetricReaderHistogramAggregation? expectedAggregation)
+    {
+        var testExecuted = false;
+
+        var configData = new Dictionary<string, string?> { [OtlpSpecConfigDefinitions.MetricsDefaultHistogramAggregationEnvVarName] = configValue };
+        var configuration = new ConfigurationBuilder()
+            .AddInMemoryCollection(configData)
+            .Build();
+
+        using var meterProvider = Sdk.CreateMeterProviderBuilder()
+            .ConfigureServices(services =>
+            {
+                services.AddSingleton<IConfiguration>(configuration);
+
+                services.PostConfigure<MetricReaderOptions>(o =>
+                {
+                    testExecuted = true;
+                    Assert.Equal(expectedAggregation, o.DefaultHistogramAggregation);
+                });
+            })
+            .AddOtlpExporter()
+            .Build();
+
+        Assert.True(testExecuted);
+    }
+
+    [Theory]
+    [InlineData("explicit_bucket_histogram", MetricReaderHistogramAggregation.ExplicitBucketHistogram)]
+    [InlineData("base2_exponential_bucket_histogram", MetricReaderHistogramAggregation.Base2ExponentialBucketHistogram)]
+    [InlineData("invalid", null)]
+    internal void TestDefaultHistogramAggregationUsingEnvVar(string configValue, MetricReaderHistogramAggregation? expectedAggregation)
+    {
+        using var scope = new EnvironmentVariableScope(OtlpSpecConfigDefinitions.MetricsDefaultHistogramAggregationEnvVarName, configValue);
+
+        var testExecuted = false;
+
+        using var meterProvider = Sdk.CreateMeterProviderBuilder()
+            .ConfigureServices(services =>
+            {
+                services.PostConfigure<MetricReaderOptions>(o =>
+                {
+                    testExecuted = true;
+                    Assert.Equal(expectedAggregation, o.DefaultHistogramAggregation);
+                });
+            })
+            .AddOtlpExporter()
+            .Build();
+
+        Assert.True(testExecuted);
+    }
+
+    [Theory]
+    [InlineData(null, false)]
+    [InlineData(MetricReaderHistogramAggregation.ExplicitBucketHistogram, false)]
+    [InlineData(MetricReaderHistogramAggregation.Base2ExponentialBucketHistogram, true)]
+    internal void TestDefaultHistogramAggregationAppliedToHistograms(MetricReaderHistogramAggregation? aggregation, bool expectExponential)
+    {
+        var exportedItems = new List<Metric>();
+
+        using (var meter = new Meter(Utils.GetCurrentMethodName()))
+        using (var meterProvider = Sdk.CreateMeterProviderBuilder()
+            .AddMeter(meter.Name)
+            .AddInMemoryExporter(exportedItems, metricReaderOptions =>
+            {
+                metricReaderOptions.DefaultHistogramAggregation = aggregation;
+            })
+            .Build())
+        {
+            var histogram = meter.CreateHistogram<long>("test_histogram");
+            histogram.Record(100);
+            histogram.Record(200);
+
+            Assert.True(meterProvider.ForceFlush());
+
+            var metric = Assert.Single(exportedItems);
+            Assert.Equal("test_histogram", metric.Name);
+
+            if (expectExponential)
+            {
+                Assert.Equal(MetricType.ExponentialHistogram, metric.MetricType);
+            }
+            else
+            {
+                Assert.Equal(MetricType.Histogram, metric.MetricType);
+            }
+        }
+    }
+
+    [Fact]
+    internal void TestDefaultHistogramAggregationOverriddenByExplicitView()
+    {
+        var exportedItems = new List<Metric>();
+
+        using (var meter = new Meter(Utils.GetCurrentMethodName()))
+        using (var meterProvider = Sdk.CreateMeterProviderBuilder()
+            .AddMeter(meter.Name)
+            .AddInMemoryExporter(exportedItems, metricReaderOptions =>
+            {
+                // Set default to exponential
+                metricReaderOptions.DefaultHistogramAggregation = MetricReaderHistogramAggregation.Base2ExponentialBucketHistogram;
+            })
+
+            // Explicit view should override the default
+            .AddView("test_histogram", new ExplicitBucketHistogramConfiguration())
+            .Build())
+        {
+            var histogram = meter.CreateHistogram<long>("test_histogram");
+            histogram.Record(100);
+            histogram.Record(200);
+
+            Assert.True(meterProvider.ForceFlush());
+
+            var metric = Assert.Single(exportedItems);
+            Assert.Equal("test_histogram", metric.Name);
+
+            // Should use explicit bucket histogram despite default being exponential
+            Assert.Equal(MetricType.Histogram, metric.MetricType);
+        }
+    }
+
+    [Fact]
+    internal void TestDefaultHistogramAggregationAppliedWhenViewsConfiguredButDontMatch()
+    {
+        var exportedItems = new List<Metric>();
+
+        using (var meter = new Meter(Utils.GetCurrentMethodName()))
+        using (var meterProvider = Sdk.CreateMeterProviderBuilder()
+            .AddMeter(meter.Name)
+            .AddInMemoryExporter(exportedItems, metricReaderOptions =>
+            {
+                // Set default to exponential
+                metricReaderOptions.DefaultHistogramAggregation = MetricReaderHistogramAggregation.Base2ExponentialBucketHistogram;
+            })
+
+            // Add a view that doesn't match our histogram
+            .AddView("counter_*", new MetricStreamConfiguration())
+            .Build())
+        {
+            var histogram = meter.CreateHistogram<long>("test_histogram");
+            histogram.Record(100);
+            histogram.Record(200);
+
+            Assert.True(meterProvider.ForceFlush());
+
+            var metric = Assert.Single(exportedItems);
+            Assert.Equal("test_histogram", metric.Name);
+
+            // Should use exponential histogram from default (views configured but didn't match)
+            Assert.Equal(MetricType.ExponentialHistogram, metric.MetricType);
+        }
+    }
+
     private static void VerifyExemplars<T>(long? longValue, double? doubleValue, bool enableExemplars, Func<T, OtlpMetrics.Exemplar?> getExemplarFunc, T state)
     {
         var exemplar = getExemplarFunc(state);