Fixes: #9259
The `Mono.toFuture()` does not propagate context to thread locals of the `CompletableFuture` consumer.
See `MonoToCompletableFuture`
* Fix `AbstractMessageProducingHandler` to convert reply `Mono` to `CompletableFuture` manually.
Use `doOnEach()` and set thread locals from the Reactor context manually around `replyFuture.complete()/completeExceptionally()`
* Add respective unit test into `WebFluxObservationPropagationTests` to ensure that same trace is used in downstream endpoints after WebFlux client reply
**Auto-cherry-pick to `6.3.x` & `6.2.x`**
4a77dbcc46