Move reactive server instrumentation out of WebFilter

Prior to this commit, the Observation instrumentation for Reactive
server applications was implemented with a `WebFilter`. This allowed to
record observations and set up a tracing context for the controller
handlers.

The limitation of this approach is that all processing happening at a
lower level is not aware of any observation. Here, the
`HttpWebHandlerAdapter` handles several interesting aspects:

* logging of HTTP requests and responses at the TRACE level
* logging of client disconnect errors
* handling of unresolved errors

With the current instrumentation, these logging statements will miss the
tracing context information. As a result, this commit deprecates the
`ServerHttpObservationFilter` in favor of a more direct instrumentation
of the `HttpWebHandlerAdapter`. This enables a more precise
instrumentattion and allows to set up the current observation earlier in
the reactor context: log statements will now contain the relevant
information.

Fixes gh-30013

framework-docs/modules/ROOT/pages/integration/observability.adoc

@@ -121,11 +121,15 @@ By default, the following `KeyValues` are created:
 [[observability.http-server.reactive]]
 === Reactive applications
 
-Applications need to configure the `org.springframework.web.filter.reactive.ServerHttpObservationFilter` reactive `WebFilter` in their application.
+Applications need to configure the `WebHttpHandlerBuilder` with a `MeterRegistry` to enable server instrumentation.
+This can be done on the `WebHttpHandlerBuilder`, as follows:
+
+include::code:HttpHandlerConfiguration[]
+
 It is using the `org.springframework.http.server.reactive.observation.DefaultServerRequestObservationConvention` by default, backed by the `ServerRequestObservationContext`.
 
-This will only record an observation as an error if the `Exception` has not been handled by the web Framework and has bubbled up to the `WebFilter`.
-Typically, all exceptions handled by Spring WebFlux's `@ExceptionHandler` and xref:web/webflux/ann-rest-exceptions.adoc[`ProblemDetail` support] will not be recorded with the observation.
+This will only record an observation as an error if the `Exception` has not been handled by an application Controller.
+Typically, all exceptions handled by Spring WebFlux's `@ExceptionHandler` and <<web.adoc#webflux-ann-rest-exceptions,`ProblemDetail` support>> will not be recorded with the observation.
 You can, at any point during request processing, set the error field on the `ObservationContext` yourself:
 
 include-code::./UserController[]