From 35cae08415d9d6d4c38aad9b27ae6acc95add09f Mon Sep 17 00:00:00 2001
From: Mikel Blanchard <mblanchard@macrosssoftware.com>
Date: Tue, 22 Mar 2022 13:01:27 -0700
Subject: [PATCH] Store immutable copies of Metric/MetricPoint inside in-memory
 exporter.

---
 .../InMemoryMetricExporter.cs                 | 78 +++++++++++++++++++
 src/OpenTelemetry/AssemblyInfo.cs             |  1 +
 src/OpenTelemetry/Metrics/HistogramBuckets.cs | 12 +++
 src/OpenTelemetry/Metrics/MetricPoint.cs      | 11 ++-
 4 files changed, 101 insertions(+), 1 deletion(-)
 create mode 100644 src/OpenTelemetry.Exporter.InMemory/InMemoryMetricExporter.cs

diff --git a/src/OpenTelemetry.Exporter.InMemory/InMemoryMetricExporter.cs b/src/OpenTelemetry.Exporter.InMemory/InMemoryMetricExporter.cs
new file mode 100644
index 00000000000..3d2fef70252
--- /dev/null
+++ b/src/OpenTelemetry.Exporter.InMemory/InMemoryMetricExporter.cs
@@ -0,0 +1,78 @@
+// <copyright file="InMemoryMetricExporter.cs" company="OpenTelemetry Authors">
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+// </copyright>
+
+using System.Collections.Generic;
+using OpenTelemetry.Metrics;
+
+namespace OpenTelemetry.Exporter
+{
+    public class InMemoryMetricExporter : BaseExporter<Metric>
+    {
+        private readonly ICollection<ExportedMetric> exportedItems;
+
+        public InMemoryMetricExporter(ICollection<ExportedMetric> exportedItems)
+        {
+            this.exportedItems = exportedItems;
+        }
+
+        public override ExportResult Export(in Batch<Metric> batch)
+        {
+            if (this.exportedItems == null)
+            {
+                return ExportResult.Failure;
+            }
+
+            foreach (var metric in batch)
+            {
+                List<MetricPoint> metricPoints = new();
+
+                foreach (ref readonly var metricPoint in metric.GetMetricPoints())
+                {
+                    metricPoints.Add(metricPoint.Copy());
+                }
+
+                this.exportedItems.Add(new ExportedMetric(metric, metricPoints));
+            }
+
+            return ExportResult.Success;
+        }
+
+        public readonly struct ExportedMetric
+        {
+            private readonly InstrumentIdentity instrumentIdentity;
+
+            internal ExportedMetric(
+                Metric metric,
+                IReadOnlyList<MetricPoint> metricPoints)
+            {
+                this.instrumentIdentity = metric.InstrumentIdentity;
+                this.MetricPoints = metricPoints;
+            }
+
+            public string Name => this.instrumentIdentity.InstrumentName;
+
+            public string Description => this.instrumentIdentity.Description;
+
+            public string Unit => this.instrumentIdentity.Unit;
+
+            public string MeterName => this.instrumentIdentity.MeterName;
+
+            public string MeterVersion => this.instrumentIdentity.MeterVersion;
+
+            public IReadOnlyList<MetricPoint> MetricPoints { get; }
+        }
+    }
+}
diff --git a/src/OpenTelemetry/AssemblyInfo.cs b/src/OpenTelemetry/AssemblyInfo.cs
index 8d6f5f821b6..786f45623a5 100644
--- a/src/OpenTelemetry/AssemblyInfo.cs
+++ b/src/OpenTelemetry/AssemblyInfo.cs
@@ -18,5 +18,6 @@
 
 [assembly: InternalsVisibleTo("OpenTelemetry.Tests" + AssemblyInfo.PublicKey)]
 [assembly: InternalsVisibleTo("OpenTelemetry.Extensions.Hosting.Tests" + AssemblyInfo.PublicKey)]
+[assembly: InternalsVisibleTo("OpenTelemetry.Exporter.InMemory" + AssemblyInfo.PublicKey)]
 [assembly: InternalsVisibleTo("DynamicProxyGenAssembly2" + AssemblyInfo.MoqPublicKey)]
 [assembly: InternalsVisibleTo("Benchmarks" + AssemblyInfo.PublicKey)]
diff --git a/src/OpenTelemetry/Metrics/HistogramBuckets.cs b/src/OpenTelemetry/Metrics/HistogramBuckets.cs
index 71e1b52eb21..626779d4901 100644
--- a/src/OpenTelemetry/Metrics/HistogramBuckets.cs
+++ b/src/OpenTelemetry/Metrics/HistogramBuckets.cs
@@ -14,6 +14,8 @@
 // limitations under the License.
 // </copyright>
 
+using System;
+
 namespace OpenTelemetry.Metrics
 {
     /// <summary>
@@ -45,6 +47,16 @@ internal HistogramBuckets(double[] explicitBounds)
 
         public Enumerator GetEnumerator() => new(this);
 
+        internal HistogramBuckets Copy()
+        {
+            HistogramBuckets copy = new HistogramBuckets(this.ExplicitBounds);
+
+            Array.Copy(this.SnapshotBucketCounts, copy.SnapshotBucketCounts, this.SnapshotBucketCounts.Length);
+            copy.SnapshotSum = this.SnapshotSum;
+
+            return copy;
+        }
+
         /// <summary>
         /// Enumerates the elements of a <see cref="HistogramBuckets"/>.
         /// </summary>
diff --git a/src/OpenTelemetry/Metrics/MetricPoint.cs b/src/OpenTelemetry/Metrics/MetricPoint.cs
index e1e2fb8e180..75f6c040e4b 100644
--- a/src/OpenTelemetry/Metrics/MetricPoint.cs
+++ b/src/OpenTelemetry/Metrics/MetricPoint.cs
@@ -28,7 +28,7 @@ public struct MetricPoint
     {
         private readonly AggregationType aggType;
 
-        private readonly HistogramBuckets histogramBuckets;
+        private HistogramBuckets histogramBuckets;
 
         // Represents temporality adjusted "value" for double/long metric types or "count" when histogram
         private MetricPointValueStorage runningValue;
@@ -541,6 +541,15 @@ internal void TakeSnapshot(bool outputDelta)
             }
         }
 
+        internal MetricPoint Copy()
+        {
+            MetricPoint copy = this;
+
+            copy.histogramBuckets = this.histogramBuckets.Copy();
+
+            return copy;
+        }
+
         [MethodImpl(MethodImplOptions.NoInlining)]
         private readonly void ThrowNotSupportedMetricTypeException(string methodName)
         {