Allow listening to TaskCanceled exceptions on HTTP client instrumentation

[tiagonapoli]

Feature Request

Is your feature request related to a problem?

I would like to add custom tags when timeouts happen on clients using the auto instrumentation, but currently when a HTTP Request is canceled and the request fails neither OnStopActivity nor OnException are called using the options.Enrich hook.

It seems this happens because when a Task is canceled the onStopActivity is called (and it handles tasks canceled):
https://github.com/open-telemetry/opentelemetry-dotnet/blob/884b224508351a65ab260c404fad5e25b341db97/src/OpenTelemetry.Instrumentation.Http/Implementation/HttpHandlerDiagnosticListener.cs#L160-L178

And since there is no response the onStopActivity will not be called:
https://github.com/open-telemetry/opentelemetry-dotnet/blob/884b224508351a65ab260c404fad5e25b341db97/src/OpenTelemetry.Instrumentation.Http/Implementation/HttpHandlerDiagnosticListener.cs#L180-L198

Describe the solution you'd like:

Maybe OnStopActivity or OnException could be called signaling the timeout?

[iliar-turdushev]

We've got a similar ask. We adopted OpenTelemetry.Instrumentation.Http 1.0.0-rc9.14 and EnrichWithHttpRequestMessage, EnrichWithHttpResponseMessage, and EnrichWithException to populate custom tags. We were using handlers EnrichWithHttpResponseMessage or EnrichWithException to execute some "final" logic supposed to be executed at request's end. But since none of these handlers were executed when a request was cancelled (timed out) we ended up with activities that didn't contain data being populated at request's end.

Executing either EnrichWithHttpResponseMessage or EnrichWithException when a request is cancelled (timed out) would solve our problem.

cc @dpk83

[dpk83]

@cijothomas @reyang