Fine tune WebFlux server logging verbosity

With this commit, WebFlux server uses warning instead of error log level
for request handling, and also just print the message instead of the
stacktrace which is mostly meaningless in reactive world.

Complementary to this change, Reactor Netty removed additional logging
as part of reactor/reactor-netty#339.

Issue: SPR-16688

Author: sdeleuze

spring-web/src/main/java/org/springframework/http/server/reactive/ReactorHttpHandlerAdapter.java

@@ -73,7 +73,7 @@ public Mono<Void> apply(HttpServerRequest request, HttpServerResponse response)
 		}
 
 		return this.httpHandler.handle(adaptedRequest, adaptedResponse)
-				.doOnError(ex -> logger.error("Handling completed with error", ex))
+				.doOnError(ex -> logger.warn("Handling completed with error: " + ex.getMessage()))
 				.doOnSuccess(aVoid -> logger.debug("Handling completed with success"));
 	}
 

spring-web/src/main/java/org/springframework/http/server/reactive/ServletHttpHandlerAdapter.java

@@ -279,7 +279,7 @@ public void onNext(Void aVoid) {
 
 		@Override
 		public void onError(Throwable ex) {
-			logger.error("Handling completed with error", ex);
+			logger.warn("Handling completed with error: " + ex.getMessage());
 			runIfAsyncNotComplete(this.asyncContext, this.isCompleted, () -> {
 				if (this.asyncContext.getResponse().isCommitted()) {
 					logger.debug("Dispatching into container to raise error");

spring-web/src/main/java/org/springframework/http/server/reactive/UndertowHttpHandlerAdapter.java

@@ -97,7 +97,7 @@ public void onNext(Void aVoid) {
 
 		@Override
 		public void onError(Throwable ex) {
-			logger.error("Handling completed with error", ex);
+			logger.warn("Handling completed with error: " + ex.getMessage());
 			if (this.exchange.isResponseStarted()) {
 				try {
 					logger.debug("Closing connection");