Reject setups with multiple observation conventions

Prior to this commit, the `WebHttpHandlerBuilder` would only configure
a custom observation convention if there is a single convention in the
application context. It would other wise use the default.

This commit aligns with the previous Spring Boot behavior where multiple
conventions setups are rejected as invalid with
`NoUniqueBeanDefinitionException`.

Fixes gh-31864

Author: bclozel

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

@@ -198,7 +198,7 @@ public static WebHttpHandlerBuilder applicationContext(ApplicationContext contex
 				.forEach(builder::httpHandlerDecorator);
 
 		context.getBeanProvider(ObservationRegistry.class).ifUnique(builder::observationRegistry);
-		context.getBeanProvider(ServerRequestObservationConvention.class).ifUnique(builder::observationConvention);
+		context.getBeanProvider(ServerRequestObservationConvention.class).ifAvailable(builder::observationConvention);
 
 		try {
 			builder.sessionManager(

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

@@ -28,6 +28,7 @@
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
+import org.springframework.beans.factory.NoUniqueBeanDefinitionException;
 import org.springframework.context.annotation.AnnotationConfigApplicationContext;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
@@ -49,6 +50,7 @@
 
 import static java.time.Duration.ofMillis;
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
 /**
  * Unit tests for {@link WebHttpHandlerBuilder}.
@@ -173,6 +175,14 @@ void observationRegistry() {
 				.that().hasLowCardinalityKeyValue("uri", "UNKNOWN");
 	}
 
+	@Test
+	void shouldRejectDuplicateObservationConvention() {
+		AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(ObservationConfig.class,
+				DuplicateConventionObservationConfig.class);
+		assertThatThrownBy(() -> WebHttpHandlerBuilder.applicationContext(applicationContext).build())
+				.isInstanceOf(NoUniqueBeanDefinitionException.class);
+	}
+
 	private static Mono<Void> writeToResponse(ServerWebExchange exchange, String value) {
 		byte[] bytes = value.getBytes(StandardCharsets.UTF_8);
 		DataBuffer buffer = DefaultDataBufferFactory.sharedInstance.wrap(bytes);
@@ -315,4 +325,14 @@ public WebHandler webHandler() {
 
 	}
 
+	@Configuration
+	static class DuplicateConventionObservationConfig {
+
+		@Bean
+		public ServerRequestObservationConvention duplicateRequestObservationConvention() {
+			return new DefaultServerRequestObservationConvention();
+		}
+
+	}
+
 }