Improve waiting for metrics

[JamesNK]

Builds on top of #4087. I thought making these changes myself would be easier than explaining them in code review.

• Add cancellation token to WaitForMeasurementsAsync.
• Simplify timeout method to pass cancellation token to WaitForMeasurementsAsync.
• Previously, waiters weren't removed from the collector if they timed out/were canceled. They are now removed.
• The TCS is now created with RunContinuationsAsynchronously to ensure awaiting user code isn't run when holding the _measurements lock.

cc @geeknoid @noahfalk @tarekgh

Microsoft Reviewers: Open in CodeFlow

[JamesNK]

I'm not sure if ObjectDisposedException is the right thing to do here. Setting the TCS to canceled might be more idiomatic. I'd need to look at what other APIs do for pending async tasks on dispose.

[geeknoid]

I decided not to use cancel semantics here since I think that would be surprising. The caller didn't cancel anything, they disposed an object.

[JamesNK]

I looked up what another type does here: HttpClient cancels any outstanding requests instead of throwing ODE - https://github.com/dotnet/runtime/blob/05e25ff8c3a623df74720f430c270eba0f1d41b0/src/libraries/System.Net.Http/src/System/Net/Http/HttpClient.cs#L706-L710

I don't care much, but I'm pretty sure most .NET APIs clean up pending tasks on dispose by canceling the tasks, not throwing ODE.

[geeknoid]

So what's the scenario where a cancellation token is useful?

[JamesNK]

So what's the scenario where a cancellation token is useful?

The developer doesn't want to wait for measurements anymore for whatever reason. Maybe they have a test that waits for results from two collectors. When one successfully returns a result they cancel waiting for the other.

[JamesNK]

@geeknoid I've rebased on main and resolved conflicts. All outstanding feedback has been addressed. Ready for review and merge?