[PoC] alternate approach for InMemoryExporter, adding selective copy to Metric (for discussion)

src/OpenTelemetry.Exporter.InMemory/InMemoryExporter.cs

@@ -16,6 +16,8 @@
 
 using System.Collections.Generic;
 
+using OpenTelemetry.Metrics;
+
 namespace OpenTelemetry.Exporter
 {
     public class InMemoryExporter<T> : BaseExporter<T>
@@ -37,7 +39,17 @@ public override ExportResult Export(in Batch<T> batch)
 
             foreach (var data in batch)
             {
-                this.exportedItems.Add(data);
+                if (data is Metric metric)
+                {
+                    // By design, The MetricApi reuses Metrics (see: MetricReader.metricsCurrentBatch).
+                    // Here we need to copy the metric to be exported to
+                    // prevent the exported instance from being updated.
+                    this.exportedItems.Add(metric.Copy() as T);
+                }
+                else
+                {
+                    this.exportedItems.Add(data);
+                }
             }
 
             return ExportResult.Success;

src/OpenTelemetry.Exporter.InMemory/OpenTelemetry.Exporter.InMemory.csproj

@@ -17,8 +17,8 @@
   </ItemGroup>
 
   <ItemGroup>
-    <Compile Include="$(RepoRoot)\src\OpenTelemetry\Internal\PeriodicExportingMetricReaderHelper.cs" Link="Includes\PeriodicExportingMetricReaderHelper.cs" />
-    <Compile Include="$(RepoRoot)\src\OpenTelemetry\Internal\ServiceProviderExtensions.cs" Link="Includes\ServiceProviderExtensions.cs" />
+    <!--<Compile Include="$(RepoRoot)\src\OpenTelemetry\Internal\PeriodicExportingMetricReaderHelper.cs" Link="Includes\PeriodicExportingMetricReaderHelper.cs" />
+    <Compile Include="$(RepoRoot)\src\OpenTelemetry\Internal\ServiceProviderExtensions.cs" Link="Includes\ServiceProviderExtensions.cs" />-->
     <Compile Include="$(RepoRoot)\src\OpenTelemetry.Api\Internal\Guard.cs" Link="Includes\Guard.cs" />
   </ItemGroup>
 

src/OpenTelemetry/AssemblyInfo.cs

@@ -17,6 +17,7 @@
 using System.Runtime.CompilerServices;
 
 [assembly: InternalsVisibleTo("OpenTelemetry.Tests" + AssemblyInfo.PublicKey)]
+[assembly: InternalsVisibleTo("OpenTelemetry.Exporter.InMemory" + AssemblyInfo.PublicKey)]
 [assembly: InternalsVisibleTo("OpenTelemetry.Exporter.Prometheus" + AssemblyInfo.PublicKey)]
 [assembly: InternalsVisibleTo("OpenTelemetry.Exporter.Prometheus.Tests" + AssemblyInfo.PublicKey)]
 [assembly: InternalsVisibleTo("OpenTelemetry.Extensions.Hosting.Tests" + AssemblyInfo.PublicKey)]

src/OpenTelemetry/Metrics/AggregatorStore.cs

@@ -80,6 +80,14 @@ internal AggregatorStore(
             }
         }
 
+        private AggregatorStore(AggregatorStore other)
+        {
+            this.batchSize = other.batchSize;
+
+            this.currentMetricPointBatch = (int[])other.currentMetricPointBatch.Clone();
+            this.metricPoints = Array.ConvertAll(other.metricPoints, metricPoints => metricPoints.Copy());
+        }
+
         private delegate void UpdateLongDelegate(long value, ReadOnlySpan<KeyValuePair<string, object>> tags);
 
         private delegate void UpdateDoubleDelegate(double value, ReadOnlySpan<KeyValuePair<string, object>> tags);
@@ -155,6 +163,8 @@ internal void SnapshotCumulative(int indexSnapshot)
         internal MetricPointsAccessor GetMetricPoints()
             => new(this.metricPoints, this.currentMetricPointBatch, this.batchSize);
 
+        internal AggregatorStore Copy() => new(this);
+
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private void InitializeZeroTagPointIfNotInitialized()
         {

src/OpenTelemetry/Metrics/HistogramBuckets.cs

@@ -52,6 +52,7 @@ internal HistogramBuckets Copy()
             HistogramBuckets copy = new HistogramBuckets(this.ExplicitBounds);
 
             Array.Copy(this.SnapshotBucketCounts, copy.SnapshotBucketCounts, this.SnapshotBucketCounts.Length);
+            Array.Copy(this.RunningBucketCounts, copy.RunningBucketCounts, this.RunningBucketCounts.Length);
             copy.SnapshotSum = this.SnapshotSum;
 
             return copy;

src/OpenTelemetry/Metrics/Metric.cs

@@ -110,6 +110,13 @@ internal Metric(
             this.InstrumentDisposed = false;
         }
 
+        private Metric(Metric other)
+        {
+            this.MetricType = other.MetricType;
+            this.InstrumentIdentity = other.InstrumentIdentity;
+            this.aggStore = other.aggStore.Copy();
+        }
+
         public MetricType MetricType { get; private set; }
 
         public AggregationTemporality Temporality { get; private set; }
@@ -147,5 +154,10 @@ internal int Snapshot()
         {
             return this.aggStore.Snapshot();
         }
+
+        internal Metric Copy()
+        {
+            return new Metric(this);
+        }
     }
 }