Prior to this commit, the ServerHttpObservationFilter would not add
the current observation as a key in the Reactor context, preventing
from being used or propagated during the HTTP exchange handling.
Also, the client instrumentation in `DefaultWebClient` would start
the observation once the request is fully formed and immutable,
preventing the context from being propagated through HTTP request
headers.
This commit fixes both uses cases now by:
* adding the current observation as a key in the reactor context
on the server side
* using the `ClientRequest.Builder` as a Carrier on the client side
Closes gh-29388
c03ccb2e6c