Fix 500 from mappings endpoint with attributed WebFlux router function

Closes gh-31784

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/web/mappings/reactive/DispatcherHandlersMappingDescriptionProvider.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2022 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.
@@ -184,8 +184,6 @@ public class DispatcherHandlersMappingDescriptionProvider implements MappingDesc
 
 		@Override
 		public void attributes(Map<String, Object> attributes) {
-			// TODO Auto-generated method stub
-			throw new UnsupportedOperationException("Auto-generated method stub");
 		}
 
 		@Override

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/web/mappings/MappingsEndpointTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2021 the original author or authors.
+ * Copyright 2012-2022 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.
@@ -164,7 +164,7 @@ class MappingsEndpointTests {
 							contextMappings, "dispatcherHandlers");
 					assertThat(dispatcherHandlers).containsOnlyKeys("webHandler");
 					List<DispatcherHandlerMappingDescription> handlerMappings = dispatcherHandlers.get("webHandler");
-					assertThat(handlerMappings).hasSize(3);
+					assertThat(handlerMappings).hasSize(4);
 				});
 	}
 
@@ -211,6 +211,11 @@ class MappingsEndpointTests {
 
 		}
 
+		@Bean
+		RouterFunction<ServerResponse> routerFunctionWithAttributes() {
+			return route(GET("/four"), (request) -> ServerResponse.ok().build()).withAttribute("test", "test");
+		}
+
 	}
 
 	@Configuration(proxyBeanMethods = false)