In Dns.RunAsync calls AfterResolution when canceled

[rokonec]

Fixes: #92045

Context

When Dns.RunAsync is canceled before it runs its action, pairing Log.AfterResolution was not called causing permanently incorrect telemetry counters value. See description of #92045

Changes

Added AfterResolution in OnlyOnCanceled registered continuation.

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[antonfirsov]

What happens if the CancellationToken is fired during the invocation of the delegate passed to prevTask.ContinueWith() (not the previously queued task)? Isn't it possible that both continuation delegates will be invoked leading to double call into AfterResolution?

Also, can we attempt to design a test for this? Idea for a "mini stress test": have a bunch parallel calls in a separate RemoteExecutor process, cancel some of them, count the ResolutionStart & ResolutionStop events.

[rokonec]

What happens if the CancellationToken is fired during the invocation of the delegate passed to prevTask.ContinueWith() (not the previously queued task)? Isn't it possible that both continuation delegates will be invoked leading to double call into AfterResolution?

Because we dont pass CT to func() prevTask.ContinueWith() if the CancellationToken is fired during the invocation of the delegate the delegate will complete and call AfterRFesolution, AND task.ContinueWith for TaskContinuationOptions.OnlyOnCanceled will not be fired as it does not care about original task CT but only about completed task status - which will not be Cancelled.

TLDR: I believe it is OK

Also, can we attempt to design a test for this? Idea for a "mini stress test": have a bunch parallel calls in a separate RemoteExecutor process, cancel some of them, count the ResolutionStart & ResolutionStop events.

Yes it will be nice to have test to validate balanced ResolutionStart & ResolutionStop, however due to various caching on DNS I have no idea how to validate that failure case when resolution is canceled after it was added into task Continuation and before this continuation has been scheduled/run. I have tried it locally and failed to came up with stable solution. If cancelation toke is canceled before call of GetHostAddressesAsync, RunAsync is not called at all, and neither After or Before resolution can possibly be called. I don't see how to overcome this in RemoteExecutor.

If this is a high concert I would recommend to handle it in isolated PR, as this requires, IMO, some refactoring and investigations.