+str #17162 add log() stage for simple logging in Flows

+ with javadsl
+ allows configuring log levels
+ allows turning off logging of certain actions completely
+ cookbook adjusted to show this instead of manual PushStage
- PENDING: preStart based impl will be faster, coming soon

akka-stream-tests/src/test/java/akka/stream/StreamTest.java

@@ -5,7 +5,9 @@
 package akka.stream;
 
 import akka.actor.ActorSystem;
+import akka.event.Logging;
 import akka.stream.javadsl.AkkaJUnitActorSystemResource;
+import akka.stream.OperationAttributes;
 
 public abstract class StreamTest {
     final protected ActorSystem system;