[sdk-metrics] Turn exemplars on by default in prerelease builds

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

@@ -55,7 +55,7 @@ static OpenTelemetry.Logs.LoggerProviderBuilderExtensions.SetResourceBuilder(thi
 static OpenTelemetry.Logs.LoggerProviderExtensions.AddProcessor(this OpenTelemetry.Logs.LoggerProvider! provider, OpenTelemetry.BaseProcessor<OpenTelemetry.Logs.LogRecord!>! processor) -> OpenTelemetry.Logs.LoggerProvider!
 static OpenTelemetry.Logs.LoggerProviderExtensions.ForceFlush(this OpenTelemetry.Logs.LoggerProvider! provider, int timeoutMilliseconds = -1) -> bool
 static OpenTelemetry.Logs.LoggerProviderExtensions.Shutdown(this OpenTelemetry.Logs.LoggerProvider! provider, int timeoutMilliseconds = -1) -> bool
-static OpenTelemetry.Metrics.MeterProviderBuilderExtensions.SetExemplarFilter(this OpenTelemetry.Metrics.MeterProviderBuilder! meterProviderBuilder, OpenTelemetry.Metrics.ExemplarFilterType exemplarFilter = OpenTelemetry.Metrics.ExemplarFilterType.TraceBased) -> OpenTelemetry.Metrics.MeterProviderBuilder!
+static OpenTelemetry.Metrics.MeterProviderBuilderExtensions.SetExemplarFilter(this OpenTelemetry.Metrics.MeterProviderBuilder! meterProviderBuilder, OpenTelemetry.Metrics.ExemplarFilterType exemplarFilter) -> OpenTelemetry.Metrics.MeterProviderBuilder!
 static OpenTelemetry.OpenTelemetryBuilderSdkExtensions.WithLogging(this OpenTelemetry.IOpenTelemetryBuilder! builder) -> OpenTelemetry.IOpenTelemetryBuilder!
 static OpenTelemetry.OpenTelemetryBuilderSdkExtensions.WithLogging(this OpenTelemetry.IOpenTelemetryBuilder! builder, System.Action<OpenTelemetry.Logs.LoggerProviderBuilder!>! configure) -> OpenTelemetry.IOpenTelemetryBuilder!
 static OpenTelemetry.OpenTelemetryBuilderSdkExtensions.WithLogging(this OpenTelemetry.IOpenTelemetryBuilder! builder, System.Action<OpenTelemetry.Logs.LoggerProviderBuilder!>? configureBuilder, System.Action<OpenTelemetry.Logs.OpenTelemetryLoggerOptions!>? configureOptions) -> OpenTelemetry.IOpenTelemetryBuilder!

src/OpenTelemetry/CHANGELOG.md

@@ -12,6 +12,13 @@
   which could have led to a measurement being dropped.
   ([#5546](https://github.com/open-telemetry/opentelemetry-dotnet/pull/5546))
 
+* **Experimental (pre-release builds only):** `Exemplar`s are now enabled
+  automatically with the default filter type set to `TraceBased`. `Exemplar`s
+  will be sampled for measurements recorded under an active trace. The default
+  filter can be changed by calling the `SetExemplarFilter` extension on
+  `MeterProviderBuilder`. Use `AlwaysOff` to disable `Exemplar`s.
+  ([#5545](https://github.com/open-telemetry/opentelemetry-dotnet/pull/5545))
+
 ## 1.8.1
 
 Released 2024-Apr-17

src/OpenTelemetry/Metrics/AggregatorStore.cs

@@ -27,7 +27,11 @@ internal sealed class AggregatorStore
     internal readonly Func<ExemplarReservoir?>? ExemplarReservoirFactory;
     internal long DroppedMeasurements = 0;
 
+#if EXPOSE_EXPERIMENTAL_FEATURES
+    private const ExemplarFilterType DefaultExemplarFilter = ExemplarFilterType.TraceBased;
+#else
     private const ExemplarFilterType DefaultExemplarFilter = ExemplarFilterType.AlwaysOff;
+#endif
     private static readonly string MetricPointCapHitFixMessage = "Consider opting in for the experimental SDK feature to emit all the throttled metrics under the overflow attribute by setting env variable OTEL_DOTNET_EXPERIMENTAL_METRICS_EMIT_OVERFLOW_ATTRIBUTE = true. You could also modify instrumentation to reduce the number of unique key/value pair combinations. Or use Views to drop unwanted tags. Or use MeterProviderBuilder.SetMaxMetricPointsPerMetricStream to set higher limit.";
     private static readonly Comparison<KeyValuePair<string, object?>> DimensionComparisonDelegate = (x, y) => x.Key.CompareTo(y.Key);
 

src/OpenTelemetry/Metrics/Builder/MeterProviderBuilderExtensions.cs

@@ -322,13 +322,13 @@ public static MeterProvider Build(this MeterProviderBuilder meterProviderBuilder
     /// Sets the <see cref="ExemplarFilterType"/> to be used for this provider
     /// which controls how measurements will be offered to exemplar reservoirs.
     /// Default provider configuration: <see
-    /// cref="ExemplarFilterType.AlwaysOff"/>.
+    /// cref="ExemplarFilterType.TraceBased"/>.
     /// </summary>
     /// <remarks>
     /// <inheritdoc cref="Exemplar"
     /// path="/remarks/para[@experimental-warning='true']"/>
-    /// <para>Note: Use <see cref="ExemplarFilterType.TraceBased"/> or <see
-    /// cref="ExemplarFilterType.AlwaysOn"/> to enable exemplars.</para>
+    /// <para>Note: Use <see cref="ExemplarFilterType.AlwaysOff"/> to disable
+    /// exemplars.</para>
     /// <para>Specification: <see
     /// href="https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#exemplarfilter"/>.</para>
     /// </remarks>
@@ -347,7 +347,7 @@ public static MeterProvider Build(this MeterProviderBuilder meterProviderBuilder
 #endif
             static MeterProviderBuilder SetExemplarFilter(
         this MeterProviderBuilder meterProviderBuilder,
-        ExemplarFilterType exemplarFilter = ExemplarFilterType.TraceBased)
+        ExemplarFilterType exemplarFilter)
     {
         meterProviderBuilder.ConfigureBuilder((sp, builder) =>
         {

test/OpenTelemetry.Tests/Metrics/MetricExemplarTests.cs

@@ -643,20 +643,30 @@ static void ValidateSecondPhase(
     }
 
     [Theory]
-    [InlineData(true)]
-    [InlineData(false)]
-    public void TestTraceBasedExemplarFilter(bool enableTracing)
+    [InlineData(true, false)]
+    [InlineData(false, false)]
+    [InlineData(true, true)]
+    [InlineData(false, true)]
+    public void TestTraceBasedExemplarFilter(bool enableTracing, bool setExemplarFilter)
     {
         var exportedItems = new List<Metric>();
 
         using var meter = new Meter($"{Utils.GetCurrentMethodName()}");
 
         var counter = meter.CreateCounter<long>("testCounter");
 
-        using var container = this.BuildMeterProvider(out var meterProvider, builder => builder
-            .AddMeter(meter.Name)
-            .SetExemplarFilter(ExemplarFilterType.TraceBased)
-            .AddInMemoryExporter(exportedItems));
+        using var container = this.BuildMeterProvider(out var meterProvider, builder =>
+        {
+            builder
+                .AddMeter(meter.Name)
+                .AddInMemoryExporter(exportedItems);
+
+            if (!IsExemplarApiExposed() || setExemplarFilter)
+            {
+                // Note: TraceBased is the SDK default for prerelease builds only
+                builder.SetExemplarFilter(ExemplarFilterType.TraceBased);
+            }
+        });
 
         if (enableTracing)
         {