Register Components metrics and tracing in WebAssembly when metrics support is explicitly enabled

[Copilot]

Register Components metrics and tracing in WebAssembly host with opt-in support

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Enable WebAssembly host to register metrics and tracing services when explicitly enabled via feature switch.

Description

WebAssembly host can now register metrics and tracing services during initialization when the System.Diagnostics.Metrics.Meter.IsSupported feature switch is enabled. Services are disabled by default (opt-in) to keep bundle sizes minimal, and can be enabled by setting the AppContext switch to true.

Changes:

• Added Microsoft.Extensions.Diagnostics reference to WebAssembly project (brought in transitively to test project)
• Registered ComponentsMetrics and ComponentsActivitySource in WebAssemblyHostBuilder.InitializeDefaultServices() behind feature switch check
• Services are disabled by default (opt-in) and can be enabled by setting the System.Diagnostics.Metrics.Meter.IsSupported AppContext switch to true
• Added test coverage verifying default behavior (telemetry not registered) and enabled behavior (telemetry registered when switch is true)

This allows developers to enable consistent telemetry collection across Blazor hosting modes when needed, while keeping WebAssembly bundle sizes minimal by default through the opt-in feature switch mechanism.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Blazor] Register Components metric and tracing by default on webassembly host.</issue_title>
<issue_description></issue_description>

<agent_instructions>Look at how this is done in the components server render mode host. Make sure that on webassembly we also register the metrics and tracingservices by default</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [Blazor] Register Components metric and tracing by default on webassembly host. #64736

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[Copilot]

Pull request overview

This PR aligns WebAssembly host telemetry registration with server-side Blazor by enabling metrics and tracing by default during host initialization. Previously, WebAssembly hosts did not automatically register ComponentsMetrics and ComponentsActivitySource, creating an inconsistency with the server render mode.

Key Changes:

• Added automatic registration of ComponentsMetrics and ComponentsActivitySource in WebAssemblyHostBuilder.InitializeDefaultServices()
• Added Microsoft.Extensions.Diagnostics dependency to both source and test projects
• Added test coverage verifying IMeterFactory registration

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/Components/WebAssembly/WebAssembly/src/Microsoft.AspNetCore.Components.WebAssembly.csproj	Added Microsoft.Extensions.Diagnostics package reference required for metrics functionality
src/Components/WebAssembly/WebAssembly/src/Hosting/WebAssemblyHostBuilder.cs	Registered ComponentsMetrics and ComponentsTracing services in InitializeDefaultServices method
src/Components/WebAssembly/WebAssembly/test/Microsoft.AspNetCore.Components.WebAssembly.Tests.csproj	Added Microsoft.Extensions.Diagnostics package reference for test project
src/Components/WebAssembly/WebAssembly/test/Hosting/WebAssemblyHostBuilderTest.cs	Added test to verify IMeterFactory registration, confirming metrics services are available

[pavelsavara]

This comes with increased download size. Please quantify the download size difference.
Please also make it opt-in (IL trimable), not always included.
Probably behind existing <MetricsSupport>true</MetricsSupport>.

[javiercn]

We have benchmarks that tell us about size regressions. We've never measured the size diff on every change for wasm stuff.

Please also make it opt-in (IL trimable), not always included.

Is there an AppContextSwitch for this?

We will want to make this work by default and be opt-out rather than opt-in since it will otherwise force people to jump through more hoops for something that we want to be easy.

[lewing]

System.Diagnostics.Metrics.Meter.IsSupported

[javiercn]

@copilot register the services behind System.Diagnostics.Metrics.Meter.IsSupported (don't use the fully qualified name, add the using if needed). Check with the microsoft docs if there is a different check for traces we could use, if there is, use that, if not, Meter.IsSupported is good enough.

[Copilot]

Registered metrics and tracing services behind the System.Diagnostics.Metrics.Meter.IsSupported feature switch check using AppContext.TryGetSwitch. Services are now opt-out (enabled by default, can be disabled via the feature switch). Commit bc57528.

[pavelsavara]

We will want to make this work by default and be opt-out rather than opt-in

I disagree
#64575 (comment)

[pavelsavara]

Thank you!