Adding log for DefaultExceptionHandler when JSON parse error ha… (lin…

…e#2131)

Motivation:

When a JSON document cannot be converted into Java object, an `IllegalArgumentException` is thrown. And this is converted to 400 status code. But it would be nice to have a log statement in `DefaultExceptionHandler` to log what is wrong with the request

Closes line#2041

core/src/main/java/com/linecorp/armeria/internal/annotation/DefaultExceptionHandler.java

@@ -16,6 +16,8 @@
 
 package com.linecorp.armeria.internal.annotation;
 
+import java.util.function.Consumer;
+
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -47,6 +49,7 @@ final class DefaultExceptionHandler implements ExceptionHandlerFunction {
     @Override
     public HttpResponse handleException(ServiceRequestContext ctx, HttpRequest req, Throwable cause) {
         if (cause instanceof IllegalArgumentException) {
+            log(log -> log.warn("{} Failed processing a request:", ctx, cause));
             return HttpResponse.of(HttpStatus.BAD_REQUEST);
         }
 
@@ -58,11 +61,15 @@ public HttpResponse handleException(ServiceRequestContext ctx, HttpRequest req,
             return ((HttpResponseException) cause).httpResponse();
         }
 
+        log(log -> log.warn("{} Unhandled exception from an annotated service:", ctx, cause));
+
+        return HttpResponse.of(HttpStatus.INTERNAL_SERVER_ERROR);
+    }
+
+    private static void log(Consumer<Logger> logConsumer) {
         if (Flags.annotatedServiceExceptionVerbosity() == ExceptionVerbosity.UNHANDLED &&
             logger.isWarnEnabled()) {
-            logger.warn("{} Unhandled exception from an annotated service:", ctx, cause);
+            logConsumer.accept(logger);
         }
-
-        return HttpResponse.of(HttpStatus.INTERNAL_SERVER_ERROR);
     }
 }