[main] Forward port metrics tests

src/libraries/Microsoft.Extensions.Diagnostics/tests/MetricsSubscriptionManagerTests.cs

@@ -0,0 +1,44 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.Metrics;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Diagnostics.Metrics;
+using Microsoft.Extensions.Options;
+using Xunit;
+
+namespace Microsoft.Extensions.Diagnostics.Tests
+{
+    public class MetricsSubscriptionManagerTests
+    {
+        [Fact]
+        public void AddMetrics_InitializesListeners()
+        {
+            var serviceCollection = new ServiceCollection();
+            serviceCollection.AddMetrics(); // Duplicate call, should not add things twice.
+            serviceCollection.AddMetrics(l => l.AddListener<FakeListener>());
+            var serviceProvider = serviceCollection.BuildServiceProvider();
+
+            // Make sure the subscription manager is started.
+            serviceProvider.GetRequiredService<IStartupValidator>().Validate();
+
+            var listeners = serviceProvider.GetRequiredService<IEnumerable<IMetricsListener>>();
+
+            var listener = Assert.Single(listeners);
+            var fakeListener = Assert.IsType<FakeListener>(listener);
+            Assert.Equal(1, fakeListener.InitializeCount);
+        }
+
+        private class FakeListener : IMetricsListener
+        {
+            public string Name => "Fake";
+            public int InitializeCount { get; private set; }
+            public MeasurementHandlers GetMeasurementHandlers() => new MeasurementHandlers();
+            public void Initialize(IObservableInstrumentsSource source) => InitializeCount++;
+            public bool InstrumentPublished(Instrument instrument, out object? userState) => throw new NotImplementedException();
+            public void MeasurementsCompleted(Instrument instrument, object? userState) => throw new NotImplementedException();
+        }
+    }
+}