Discuss: Should HttpForwarder emit ForwarderStage.SendAsyncStop on error?

### What should we add or change to make your life better?
This issue is meant more to document current behavior and to create a place to discuss, rather than necessarily to suggest a change.

`HttpForwarder` today emits a `ForwarderStage.SendAsyncStart` event when calling the underly9ing `httpClient.SendAsync`, but it only emits the paired `ForwarderStage.SendAsyncStop` upon success. Failures / cancellations end up without a matching Stop event.

While this is not a blocker (e.g. consumers can leverage the `ForwarderStop` event to know that the entire proxy is done, including the `httpClient.SendAsync` call), not having the `SendAsyncStop` event may be unexpected and might violate the [Principle of Least Astonishment](https://en.wikipedia.org/wiki/Principle_of_least_astonishment).

### Why is this important to you?
We only found out about this after running YARP in production for a while and noticing our telemetry wasn't coming precisely how we had designed it. Other people may similarly be caught by surprise, leading to some rework.