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

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

@@ -28,6 +28,7 @@ import org.junit.jupiter.api.Test;
 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 org.springframework.web.testfixture.http.server.reactive.MockServerHttpRe
 
 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 @@ public class WebHttpHandlerBuilderTests {
 				.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 class WebHttpHandlerBuilderTests {
 
 	}
 
+	@Configuration
+	static class DuplicateConventionObservationConfig {
+
+		@Bean
+		public ServerRequestObservationConvention duplicateRequestObservationConvention() {
+			return new DefaultServerRequestObservationConvention();
+		}
+
+	}
+
 }