Prior to this commit, regressions were introduced with gh-31417:
1. the observation keyvalues would be inconsistent with the HTTP response
2. the observation scope would not cover all controller handlers, causing
traceIds to be missing
The first issue is caused by the fact that in case of error signals, the
observation was stopped before the response was fully committed, which
means further processing could happen and update the response status.
This commit delays the stop event until the response is committed in
case of errors.
The second problem is caused by the change from a `contextWrite`
operator to using the `tap` operator with a `SignalListener`. The
observation was started in the `doOnSubscription` callback, which is too
late in some cases. If the WebFlux controller handler is synchronous
non-blocking, the execution of the handler is performed before the
subscription happens. This means that for those handlers, the
observation was not started, even if the current observation was
present in the reactor context. This commit changes the
`doOnSubscription` to `doFirst` to ensure that the observation is
started at the right time.
Fixes gh-31703
Fixes gh-31706
35fcbae8c6