Prior to this commit, the server observability support would create a
cycle in Java packages.
This commit refactors the current arrangement to solve this by:
* "flattening" the reactive HTTP instrumentation; this removes the
dependency to the `ServerWebExchange` and `PathPattern` types
* moving the `observation` package under
`org.springframework.http.server` and
`org.springframework.http.server.reactive`
See gh-29477
1ad7cc3702