Polish Observability instrumentation

This commit removes the scoping of `RestTemplate` requests, since no
user code is meant to run during HTTP client exchanges.

This also polishes the reactive `WebFilter` to directly use the
specific enum when creating the observation.

Author: bclozel

spring-web/src/main/java/org/springframework/web/client/RestTemplate.java

@@ -849,9 +849,7 @@ protected <T> T doExecute(URI url, @Nullable String uriTemplate, @Nullable HttpM
 			if (requestCallback != null) {
 				requestCallback.doWithRequest(request);
 			}
-			try (Observation.Scope scope = observation.openScope()) {
-				response = request.execute();
-			}
+			response = request.execute();
 			observationContext.setResponse(response);
 			handleResponse(url, method, response);
 			return (responseExtractor != null ? responseExtractor.extractData(response) : null);

spring-web/src/main/java/org/springframework/web/observation/reactive/HttpRequestsObservationWebFilter.java

@@ -97,8 +97,8 @@ public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
 	}
 
 	private Publisher<Void> filter(ServerWebExchange exchange, HttpRequestsObservationContext observationContext, Mono<Void> call) {
-		Observation observation = Observation.createNotStarted(this.observationConvention, DEFAULT_OBSERVATION_CONVENTION,
-				observationContext, this.observationRegistry);
+		Observation observation = HttpRequestsObservation.HTTP_REQUESTS.createNotStarted(this.observationConvention,
+				DEFAULT_OBSERVATION_CONVENTION, () -> observationContext, this.observationRegistry);
 		observation.start();
 		return call.doOnEach(signal -> {
 					Throwable throwable = signal.getThrowable();