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/src/main/java/org/springframework/docs/integration/observability/httpserver/reactive/HttpHandlerConfiguration.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2002-2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.docs.integration.observability.httpserver.reactive;
+
+import io.micrometer.observation.ObservationRegistry;
+
+import org.springframework.context.ApplicationContext;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.http.server.reactive.HttpHandler;
+import org.springframework.web.server.adapter.WebHttpHandlerBuilder;
+
+@Configuration(proxyBeanMethods = false)
+public class HttpHandlerConfiguration {
+
+	private final ApplicationContext applicationContext;
+
+	public HttpHandlerConfiguration(ApplicationContext applicationContext) {
+		this.applicationContext = applicationContext;
+	}
+
+	@Bean
+	public HttpHandler httpHandler(ObservationRegistry registry) {
+		return WebHttpHandlerBuilder.applicationContext(this.applicationContext)
+				.observationRegistry(registry)
+				.build();
+	}
+}

framework-docs/src/main/java/org/springframework/docs/integration/observability/httpserver/reactive/UserController.java

@@ -17,9 +17,9 @@
 package org.springframework.docs.integration.observability.httpserver.reactive;
 
 import org.springframework.http.ResponseEntity;
+import org.springframework.http.server.reactive.observation.ServerRequestObservationContext;
 import org.springframework.stereotype.Controller;
 import org.springframework.web.bind.annotation.ExceptionHandler;
-import org.springframework.web.filter.reactive.ServerHttpObservationFilter;
 import org.springframework.web.server.ServerWebExchange;
 
 @Controller
@@ -28,7 +28,7 @@ public class UserController {
 	@ExceptionHandler(MissingUserException.class)
 	ResponseEntity<Void> handleMissingUser(ServerWebExchange exchange, MissingUserException exception) {
 		// We want to record this exception with the observation
-		ServerHttpObservationFilter.findObservationContext(exchange)
+		ServerRequestObservationContext.findCurrent(exchange)
 				.ifPresent(context -> context.setError(exception));
 		return ResponseEntity.notFound().build();
 	}