AddMetrics can initialize MetricsSubscriptionManager multiple times

[JamesNK]

Description

Calling AddMetrics initializes the subscription manager:

runtime/src/libraries/Microsoft.Extensions.Diagnostics/src/Metrics/MetricsServiceExtensions.cs

Lines 33 to 35 in 64243bb

	// Make sure the subscription manager is started when the host starts.
	// The host will trigger options validation.
	services.AddOptions<NoOpOptions>().Configure<MetricsSubscriptionManager>((_, manager) => manager.Initialize()).ValidateOnStart();

However, the way this is done isn't resilient to multiple calls to AddMetrics by app or library code.

Listeners are initialized multiple times:

runtime/src/libraries/Microsoft.Extensions.Diagnostics/src/Metrics/MetricsSubscriptionManager.cs

Lines 30 to 36 in 64243bb

	public void Initialize()
	{
	foreach (var listener in _listeners)
	{
	listener.Initialize();
	}
	}

Reproduction Steps

services.AddMetrics();
services.AddMetrics();

Expected behavior

Multiple IConfigurateOptions<MetricsServiceExtensions+NoOpOptions> aren't registered in DI. MetricsSubscriptionManager is initialized once.

Actual behavior

Multiple IConfigurateOptions<MetricsServiceExtensions+NoOpOptions> is registered in DI. Each will instance will call initialize on MetricsSubscriptionManager.

I noticed this when the AddMvc_Twice_DoesNotAddDuplicates test in aspnetcore started throwing because there were duplicate config options.

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

No response

[Tratcher]

This doesn't seem critical for rc1, there aren't currently any implementations of IMetricsListener.Initialize other than the console, and the early adopters can mitigate this in their own code until rc2.