Prior to this commit, the request execution observation would have a
context of type `RequestReplyReceiverContext` to directly deal with
tracing propagation at the transport level. This approach doesn't work
anymore as the parent observation is not properly set on the resulting
trace, even if it is manually set in the instrumentation.
This commit revisits the request observation setup and turns its context
into a regular `Observation.Context`. Tracing propagation should be
dealt with directly at the transport level by an underlying observation.
This is the case already for Spring Framework HTTP server observations.
As a result, the `PropagationWebGraphQlInterceptor` is deprecated with
no replacement and should not be used anymore.
Fixes gh-675
4e5aaeddab