Add a failing unit test for InMemoryExporter<Metric>

test/OpenTelemetry.Tests/Metrics/InMemoryExporterTests.cs

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+// <copyright file="InMemoryExporterTests.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 System.Diagnostics.Metrics;
+using OpenTelemetry.Exporter;
+using Xunit;
+
+namespace OpenTelemetry.Metrics.Tests
+{
+    public class InMemoryExporterTests
+    {
+        [Fact(Skip = "To be run after https://github.com/open-telemetry/opentelemetry-dotnet/issues/2361 is fixed")]
+        public void InMemoryExporterShouldDeepCopyMetricPoints()
+        {
+            var meter = new Meter("InMemoryExporterTests", "1.0");
+
+            var exportedItems = new List<Metric>();
+            using var inMemoryReader = new BaseExportingMetricReader(new InMemoryExporter<Metric>(exportedItems))
+            {
+                PreferredAggregationTemporality = AggregationTemporality.Delta,
+            };
+
+            using var meterProvider = Sdk.CreateMeterProviderBuilder()
+            .AddSource("InMemoryExporterTests")
+            .AddMetricReader(inMemoryReader)
+            .Build();
+
+            var counter = meter.CreateCounter<long>("meter");
+
+            // Emit 10 for the MetricPoint with a single key-vaue pair: ("tag1", "value1")
+            counter.Add(10, new KeyValuePair<string, object>("tag1", "value1"));
+
+            // Pull metric data from AggregatorStore
+            inMemoryReader.Collect();
+
+            var metric = exportedItems[0]; // Only one Metric object is added to the collection at this point
+            var metricPointsEnumerator = metric.GetMetricPoints().GetEnumerator();
+            Assert.True(metricPointsEnumerator.MoveNext()); // One MetricPoint is emitted for the Metric
+            ref var metricPointForFirstExport = ref metricPointsEnumerator.Current;
+            Assert.Equal(10, metricPointForFirstExport.LongValue);
+
+            // Emit 25 for the MetricPoint with a single key-vaue pair: ("tag1", "value1")
+            counter.Add(25, new KeyValuePair<string, object>("tag1", "value1"));
+
+            // Pull metric data from AggregatorStore
+            inMemoryReader.Collect();
+
+            metric = exportedItems[1]; // Second Metric object is added to the collection at this point
+            metricPointsEnumerator = metric.GetMetricPoints().GetEnumerator();
+            Assert.True(metricPointsEnumerator.MoveNext()); // One MetricPoint is emitted for the Metric
+            var metricPointForSecondExport = metricPointsEnumerator.Current;
+            Assert.Equal(25, metricPointForSecondExport.LongValue);
+
+            // MetricPoint.LongValue for the first exporter metric should still be 10
+            Assert.Equal(10, metricPointForFirstExport.LongValue);
+        }
+    }
+}