Prior to this commit, the Actuator instrumentation for WebFlux servers
would not record metrics in two cases:

* the client disconnects before the response has been sent
* a server timeout is triggered before the response is sent

This commit improves the existing instrumentation to record metrics in
these cases. Since the causes of timeouts/disconnections can vary a lot,
the chosen "outcome" tag for metrics is "UNKNOWN".

Closes gh-23606