To propagate an `Observation` from reactive stream (e.g. WebFlux)
we have to capture its context and set it into the current thread scope.
* Add a `io.micrometer:context-propagation` dependency to support reactive context propagation
* Populate a `parentObservation` in the `IntegrationObservation.PRODUCER.observation()`
since it is not available for tracing on `Observation.onStart()`.
Might be tentative until upcoming fix in Micrometer Observation
* Populate from reactive context in the `WebFluxInboundEndpoint` where we use just `send()` operation downstream
* Populate from reactive context in the `MessagingGatewaySupport` where we use `send()` operation downstream or `FluxMessageChannel.subscribeTo()`
* Use `contextCapture()` in the `FluxMessageChannel` to gather a `ThreadLocal` info into a Reactor context
and then set back to `ThreadLocal` in the `transformDeferredContextual()` which really happens on a different thread
* Verify a trace propagation from WebFlux to an integration flow via Brave instrumentation in the `WebFluxObservationPropagationTests`
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