Auto-detect observability beans in WebHttpHandlerBuilder

This commit ensures that both `ObservationRegsitry` and
`ServerRequestObservationConvention` beans are automatically detected in
the application context if they are unique. This aligns with the
existing behavior for all other builder methods.

Closes gh-31205

Author: bclozel

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

@@ -162,6 +162,10 @@ public static WebHttpHandlerBuilder webHandler(WebHandler webHandler) {
 	 * ordered.
 	 * <li>{@link HttpHandlerDecoratorFactory} [0..N] -- detected by type and
 	 * ordered.
+	 * <li>{@link ObservationRegistry} -- detected by type and
+	 * configured if unique.
+	 * <li>{@link ServerRequestObservationConvention} -- detected by type and
+	 * configured if unique.
 	 * <li>{@link WebSessionManager} [0..1] -- looked up by the name
 	 * {@link #WEB_SESSION_MANAGER_BEAN_NAME}.
 	 * <li>{@link ServerCodecConfigurer} [0..1] -- looked up by the name
@@ -193,6 +197,9 @@ public static WebHttpHandlerBuilder applicationContext(ApplicationContext contex
 				.orderedStream()
 				.forEach(builder::httpHandlerDecorator);
 
+		context.getBeanProvider(ObservationRegistry.class).ifUnique(builder::observationRegistry);
+		context.getBeanProvider(ServerRequestObservationConvention.class).ifUnique(builder::observationConvention);
+
 		try {
 			builder.sessionManager(
 					context.getBean(WEB_SESSION_MANAGER_BEAN_NAME, WebSessionManager.class));

spring-web/src/test/java/org/springframework/web/server/adapter/WebHttpHandlerBuilderTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * 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.
@@ -22,6 +22,8 @@
 import java.util.function.BiFunction;
 import java.util.function.Function;
 
+import io.micrometer.observation.tck.TestObservationRegistry;
+import io.micrometer.observation.tck.TestObservationRegistryAssert;
 import org.junit.jupiter.api.Test;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
@@ -36,6 +38,8 @@
 import org.springframework.http.server.reactive.HttpHandler;
 import org.springframework.http.server.reactive.HttpHandlerDecoratorFactory;
 import org.springframework.http.server.reactive.ServerHttpRequest;
+import org.springframework.http.server.reactive.observation.DefaultServerRequestObservationConvention;
+import org.springframework.http.server.reactive.observation.ServerRequestObservationConvention;
 import org.springframework.web.server.ServerWebExchange;
 import org.springframework.web.server.WebExceptionHandler;
 import org.springframework.web.server.WebFilter;
@@ -50,6 +54,7 @@
  * Unit tests for {@link WebHttpHandlerBuilder}.
  *
  * @author Rossen Stoyanchev
+ * @author Brian Clozel
  */
 public class WebHttpHandlerBuilderTests {
 
@@ -155,6 +160,19 @@ void httpHandlerDecoratorFactoryBeans() {
 		assertThat(headerValue.apply("decoratorC")).isLessThan(headerValue.apply("decoratorB"));
 	}
 
+	@Test
+	void observationRegistry() {
+		AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(ObservationConfig.class);
+		HttpHandler handler = WebHttpHandlerBuilder.applicationContext(applicationContext).build();
+
+		MockServerHttpResponse response = new MockServerHttpResponse();
+		handler.handle(MockServerHttpRequest.get("/").build(), response).block();
+
+		TestObservationRegistry observationRegistry = applicationContext.getBean(TestObservationRegistry.class);
+		TestObservationRegistryAssert.assertThat(observationRegistry).hasObservationWithNameEqualTo("http.server.requests")
+				.that().hasLowCardinalityKeyValue("uri", "UNKNOWN");
+	}
+
 	private static Mono<Void> writeToResponse(ServerWebExchange exchange, String value) {
 		byte[] bytes = value.getBytes(StandardCharsets.UTF_8);
 		DataBuffer buffer = DefaultDataBufferFactory.sharedInstance.wrap(bytes);
@@ -277,4 +295,24 @@ public WebHandler webHandler() {
 		}
 	}
 
+	@Configuration
+	static class ObservationConfig {
+
+		@Bean
+		public TestObservationRegistry testObservationRegistry() {
+			return TestObservationRegistry.create();
+		}
+
+		@Bean
+		public ServerRequestObservationConvention requestObservationConvention() {
+			return new DefaultServerRequestObservationConvention();
+		}
+
+		@Bean
+		public WebHandler webHandler() {
+			return exchange -> exchange.getResponse().setComplete();
+		}
+
+	}
+
 }