Fix and tests for multiple metric readers

src/OpenTelemetry/CHANGELOG.md

@@ -6,6 +6,10 @@ Notes](../../RELEASENOTES.md).
 
 ## Unreleased
 
+* Added a verification to ensure that a `MetricReader` can only be registered
+  to a single `MeterProvider`, as required by the OpenTelemetry specification.
+  ([#6458](https://github.com/open-telemetry/opentelemetry-dotnet/pull/6458))
+
 * Added `FormatMessage` configuration option to self-diagnostics feature. When
   set to `true` (default is false), log messages will be formatted by replacing
   placeholders with actual parameter values for improved readability.

src/OpenTelemetry/Metrics/Reader/MetricReader.cs

@@ -218,6 +218,11 @@ public void Dispose()
 
     internal virtual void SetParentProvider(BaseProvider parentProvider)
     {
+        if (this.parentProvider != null && this.parentProvider != parentProvider)
+        {
+            throw new NotSupportedException("A MetricReader must not be registered with multiple MeterProviders.");
+        }
+
         this.parentProvider = parentProvider;
     }
 

test/OpenTelemetry.Tests/Metrics/MultipleReadersTests.cs

@@ -3,13 +3,104 @@
 
 using System.Diagnostics.Metrics;
 using Microsoft.Extensions.DependencyInjection;
+using OpenTelemetry.Exporter;
 using OpenTelemetry.Tests;
 using Xunit;
 
 namespace OpenTelemetry.Metrics.Tests;
 
 public class MultipleReadersTests
 {
+    [Fact]
+    public void ReaderCannotBeRegisteredMoreThanOnce()
+    {
+        var exportedItems = new List<Metric>();
+        using var exporter = new InMemoryExporter<Metric>(exportedItems);
+        using var reader = new BaseExportingMetricReader(exporter);
+
+        using var meter = new Meter($"{Utils.GetCurrentMethodName()}");
+
+        var meterProviderBuilder1 = Sdk.CreateMeterProviderBuilder()
+            .AddMeter(meter.Name)
+            .AddReader(reader);
+        var meterProviderBuilder2 = Sdk.CreateMeterProviderBuilder()
+            .AddMeter(meter.Name)
+            .AddReader(reader);
+
+        using var meterProvider1 = meterProviderBuilder1.Build();
+        Assert.Throws<NotSupportedException>(() => meterProviderBuilder2.Build());
+    }
+
+    [Fact]
+    public void MultipleReadersOneCollectsIndependently()
+    {
+        var exportedItems1 = new List<Metric>();
+        var exportedItems2 = new List<Metric>();
+
+        using var exporter1 = new InMemoryExporter<Metric>(exportedItems1);
+        using var exporter2 = new InMemoryExporter<Metric>(exportedItems2);
+        using var reader1 = new BaseExportingMetricReader(exporter1);
+        using var reader2 = new BaseExportingMetricReader(exporter2);
+
+        using var meter = new Meter($"{Utils.GetCurrentMethodName()}");
+        var counter = meter.CreateCounter<long>("counter");
+
+        var meterProviderBuilder = Sdk.CreateMeterProviderBuilder()
+            .AddMeter(meter.Name)
+            .AddReader(reader1)
+            .AddReader(reader2);
+
+        using var meterProvider = meterProviderBuilder.Build();
+
+        counter.Add(1);
+
+        reader1.Collect();
+
+        Assert.Single(exportedItems1);
+        Assert.Empty(exportedItems2);
+    }
+
+    [Fact]
+    public void MultipleReadersDifferentTemporality()
+    {
+        var exportedItems1 = new List<Metric>();
+        var exportedItems2 = new List<Metric>();
+
+        using var exporter1 = new InMemoryExporter<Metric>(exportedItems1);
+        using var exporter2 = new InMemoryExporter<Metric>(exportedItems2);
+        using var reader1 = new BaseExportingMetricReader(exporter1);
+        using var reader2 = new BaseExportingMetricReader(exporter2);
+        reader1.TemporalityPreference = MetricReaderTemporalityPreference.Delta;
+        reader2.TemporalityPreference = MetricReaderTemporalityPreference.Cumulative;
+
+        using var meter = new Meter($"{Utils.GetCurrentMethodName()}");
+        var counter = meter.CreateCounter<long>("counter");
+
+        var meterProviderBuilder = Sdk.CreateMeterProviderBuilder()
+            .AddMeter(meter.Name)
+            .AddReader(reader1)
+            .AddReader(reader2);
+
+        using var meterProvider = meterProviderBuilder.Build();
+
+        counter.Add(1);
+
+        reader1.Collect();
+        reader2.Collect();
+
+        AssertLongSumValueForMetric(exportedItems1[0], 1);
+        AssertLongSumValueForMetric(exportedItems2[0], 1);
+
+        exportedItems1.Clear();
+
+        counter.Add(10);
+        reader1.Collect();
+        reader2.Collect();
+
+        AssertLongSumValueForMetric(exportedItems1[0], 10);
+        AssertLongSumValueForMetric(exportedItems2[0], 11);
+    }
+
     [Theory]
     [InlineData(MetricReaderTemporalityPreference.Delta, false)]
     [InlineData(MetricReaderTemporalityPreference.Delta, true)]