Remove observation context from ClientRequest

Prior to this commit, gh-31609 added the current observation context as
a request attribute for `WebClient` calls. While it was not confirmed as
the main cause, this feature was linked to several reports of memory
leaks. This would indeed attach more memory to the request object graph
at runtime - although it shouldn't prevent its collection by the GC.

This commit removes this feature and instead developers should get the
current observation from the reactor context if they wish to interact
with it.

Closes gh-32198

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/WebClientObservationTests.java

@@ -148,23 +148,6 @@ class WebClientObservationTests {
 		verifyAndGetRequest();
 	}
 
-	@Test
-	void setsCurrentObservationContextAsRequestAttribute() {
-		ExchangeFilterFunction assertionFilter = (request, chain) -> {
-			Optional<ClientRequestObservationContext> observationContext = ClientRequestObservationContext.findCurrent(request);
-			assertThat(observationContext).isPresent();
-			return chain.exchange(request).contextWrite(context -> {
-				Observation currentObservation = context.get(ObservationThreadLocalAccessor.KEY);
-				assertThat(currentObservation.getContext()).isEqualTo(observationContext.get());
-				return context;
-			});
-		};
-		this.builder.filter(assertionFilter).build().get().uri("/resource/{id}", 42)
-				.retrieve().bodyToMono(Void.class)
-				.block(Duration.ofSeconds(10));
-		verifyAndGetRequest();
-	}
-
 	@Test
 	void recordsObservationWithResponseDetailsWhenFilterFunctionErrors() {
 		ExchangeFilterFunction errorFunction = (req, next) -> next.exchange(req).then(Mono.error(new IllegalStateException()));