Apply instrumentation fixes to HttpWebHandlerAdapter

Since the Spring WebFlux HTTP server instrumentation has been moved from
the `WebFilter` to the `HttpWebHandlerAdapter`, we need to apply similar
changes there.

See gh-31417

spring-web/src/main/java/org/springframework/web/server/adapter/HttpWebHandlerAdapter.java

@@ -17,14 +17,16 @@
 package org.springframework.web.server.adapter;
 
 import java.util.Set;
+import java.util.concurrent.atomic.AtomicBoolean;
 
 import io.micrometer.observation.Observation;
 import io.micrometer.observation.ObservationRegistry;
 import io.micrometer.observation.contextpropagation.ObservationThreadLocalAccessor;
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
-import org.reactivestreams.Publisher;
+import reactor.core.observability.DefaultSignalListener;
 import reactor.core.publisher.Mono;
+import reactor.util.context.Context;
 
 import org.springframework.context.ApplicationContext;
 import org.springframework.core.NestedExceptionUtils;
@@ -302,7 +304,9 @@ public class HttpWebHandlerAdapter extends WebHandlerDecorator implements HttpHa
 				ServerRequestObservationContext.CURRENT_OBSERVATION_CONTEXT_ATTRIBUTE, observationContext);
 
 		return getDelegate().handle(exchange)
-				.transformDeferred(call -> transform(exchange, observationContext, call))
+				.doOnSuccess(aVoid -> logResponse(exchange))
+				.onErrorResume(ex -> handleUnresolvedError(exchange, observationContext, ex))
+				.tap(() -> new ObservationSignalListener(observationContext))
 				.then(exchange.cleanupMultipart())
 				.then(Mono.defer(response::setComplete));
 	}
@@ -324,42 +328,6 @@ public class HttpWebHandlerAdapter extends WebHandlerDecorator implements HttpHa
 		return "HTTP " + request.getMethod() + " \"" + request.getPath() + query + "\"";
 	}
 
-	private Publisher<Void> transform(ServerWebExchange exchange, ServerRequestObservationContext observationContext, Mono<Void> call) {
-		Observation observation = ServerHttpObservationDocumentation.HTTP_REACTIVE_SERVER_REQUESTS.observation(
-				this.observationConvention, DEFAULT_OBSERVATION_CONVENTION, () -> observationContext, this.observationRegistry);
-		observation.start();
-		return call
-				.doOnSuccess(aVoid -> {
-					logResponse(exchange);
-					stopObservation(observation, exchange);
-				})
-				.onErrorResume(ex -> handleUnresolvedError(exchange, observationContext, ex))
-				.doOnCancel(() -> cancelObservation(observationContext, observation))
-				.contextWrite(context -> context.put(ObservationThreadLocalAccessor.KEY, observation));
-	}
-
-	private void stopObservation(Observation observation, ServerWebExchange exchange) {
-		Throwable throwable = exchange.getAttribute(ExceptionHandlingWebHandler.HANDLED_WEB_EXCEPTION);
-		if (throwable != null) {
-			observation.error(throwable);
-		}
-		ServerHttpResponse response = exchange.getResponse();
-		if (response.isCommitted()) {
-			observation.stop();
-		}
-		else {
-			response.beforeCommit(() -> {
-				observation.stop();
-				return Mono.empty();
-			});
-		}
-	}
-
-	private void cancelObservation(ServerRequestObservationContext observationContext, Observation observation) {
-		observationContext.setConnectionAborted(true);
-		observation.stop();
-	}
-
 	private void logResponse(ServerWebExchange exchange) {
 		LogFormatUtils.traceDebug(logger, traceOn -> {
 			HttpStatusCode status = exchange.getResponse().getStatusCode();
@@ -415,4 +383,66 @@ public class HttpWebHandlerAdapter extends WebHandlerDecorator implements HttpHa
 		return DISCONNECTED_CLIENT_EXCEPTIONS.contains(ex.getClass().getSimpleName());
 	}
 
+	private final class ObservationSignalListener extends DefaultSignalListener<Void> {
+
+		private final ServerRequestObservationContext observationContext;
+
+		private final Observation observation;
+
+		private AtomicBoolean observationRecorded = new AtomicBoolean();
+
+		public ObservationSignalListener(ServerRequestObservationContext observationContext) {
+			this.observationContext = observationContext;
+			this.observation = ServerHttpObservationDocumentation.HTTP_REACTIVE_SERVER_REQUESTS.observation(observationConvention,
+					DEFAULT_OBSERVATION_CONVENTION, () -> observationContext, observationRegistry);
+		}
+
+		@Override
+		public void doOnSubscription() throws Throwable {
+			this.observation.start();
+		}
+
+		@Override
+		public Context addToContext(Context originalContext) {
+			return originalContext.put(ObservationThreadLocalAccessor.KEY, this.observation);
+		}
+
+		@Override
+		public void doOnCancel() throws Throwable {
+			if (this.observationRecorded.compareAndSet(false, true)) {
+				this.observationContext.setConnectionAborted(true);
+				this.observation.stop();
+			}
+		}
+
+		@Override
+		public void doOnComplete() throws Throwable {
+			if (this.observationRecorded.compareAndSet(false, true)) {
+				ServerHttpResponse response = this.observationContext.getResponse();
+				Throwable throwable = (Throwable) this.observationContext.getAttributes()
+						.get(ExceptionHandlingWebHandler.HANDLED_WEB_EXCEPTION);
+				if (throwable != null) {
+					this.observation.error(throwable);
+				}
+				if (response.isCommitted()) {
+					this.observation.stop();
+				}
+				else {
+					response.beforeCommit(() -> {
+						this.observation.stop();
+						return Mono.empty();
+					});
+				}
+			}
+		}
+
+		@Override
+		public void doOnError(Throwable error) throws Throwable {
+			if (this.observationRecorded.compareAndSet(false, true)) {
+				this.observationContext.setError(error);
+				this.observation.stop();
+			}
+		}
+	}
+
 }