Serialize CLI host tests to fix sample_rate race (#17450)

tests/Aspire.Cli.Tests/CliBootstrapTests.cs

@@ -18,6 +18,7 @@ namespace Aspire.Cli.Tests;
 /// <see cref="IIdentityChannelReader"/>, registered in DI by
 /// <see cref="Aspire.Cli.Program.BuildApplicationAsync"/>.
 /// </summary>
+[Collection(CliHostTestCollection.Name)]
 public class CliBootstrapTests(ITestOutputHelper outputHelper)
 {
     private static readonly string[] s_fixedChannels = ["stable", "staging", "daily", "local"];

tests/Aspire.Cli.Tests/CliHostTestCollection.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.
+
+namespace Aspire.Cli.Tests;
+
+/// <summary>
+/// Collection definition that disables parallel execution for tests which
+/// build the real Aspire CLI host (via <see cref="Aspire.Cli.Program.Main"/>
+/// or <see cref="Aspire.Cli.Program.BuildApplicationAsync"/>) or otherwise
+/// create a live <see cref="Aspire.Cli.Telemetry.TelemetryManager"/> with the
+/// Azure Monitor exporter enabled in-process.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Tracks <see href="https://github.com/microsoft/aspire/issues/17450"/>.
+/// </para>
+/// <para>
+/// Azure.Monitor.OpenTelemetry.Exporter uses <c>RateLimitedSampler</c> by default,
+/// which returns a <c>SamplingResult</c> containing a <c>microsoft.sample_rate</c>
+/// attribute once its adaptive sampling state has matured (~200ms after the
+/// sampler is constructed). OpenTelemetry's <c>TracerProviderSdk</c> then writes
+/// those attributes into <c>ActivityCreationOptions.SamplingTags</c> using a hard
+/// <see cref="System.Diagnostics.ActivityTagsCollection.Add(string, object?)"/>
+/// (no <c>TryAdd</c>). <c>ActivitySource.CreateActivity</c> reuses the SAME
+/// <c>ActivityCreationOptions</c> instance across every registered listener, so
+/// when two listeners on the <c>Aspire.Cli.Reported</c> source both run samplers
+/// that emit <c>microsoft.sample_rate</c>, the second <c>Add</c> throws
+/// <see cref="System.InvalidOperationException"/> ("The collection already
+/// contains item with same key 'microsoft.sample_rate'").
+/// </para>
+/// <para>
+/// Aspire CLI production never has more than one live <see cref="Aspire.Cli.Telemetry.TelemetryManager"/>
+/// (registered as a DI singleton), so the bug is invisible at runtime. xUnit v3
+/// runs test classes in parallel by default, however, which lets two host-building
+/// tests race in-process. Placing every such test class in this collection
+/// serializes their execution and keeps at most one Azure Monitor <c>TracerProvider</c>
+/// alive at a time, eliminating the duplicate sampling-tag race.
+/// </para>
+/// </remarks>
+[CollectionDefinition(Name, DisableParallelization = true)]
+public sealed class CliHostTestCollection
+{
+    public const string Name = "CliHostTests";
+}

tests/Aspire.Cli.Tests/CliSmokeTests.cs

@@ -6,6 +6,7 @@
 
 namespace Aspire.Cli.Tests;
 
+[Collection(CliHostTestCollection.Name)]
 public class CliSmokeTests(ITestOutputHelper outputHelper)
 {
     private static readonly RemoteInvokeOptions s_remoteInvokeOptions = new()

tests/Aspire.Cli.Tests/Telemetry/TelemetryConfigurationTests.cs

@@ -15,6 +15,7 @@
 
 namespace Aspire.Cli.Tests.Telemetry;
 
+[Collection(CliHostTestCollection.Name)]
 public class TelemetryConfigurationTests
 {
     private static async Task<IHost> BuildHostAsync(Dictionary<string, string?>? config = null)