+act #3671 Implementing support for MDC values defined by the application. Fixes #3671.

* Added LogEvent subclasses with new field for transporting the MDC custom values.
* Slf4jLogger now takes MDC values from new LogEvent field, puts all in MDC before appending the log, and removes all after.
* New trait DiagnosticLoggingAdapter was introduced, which extends LoggingAdapter and adds MDC support with methods to get, set and clear MDC values.
* New factory method added to Logging for getting loggers with MDC support.
* BusLogging was changed to create new LogEvents including the MDC values.
* Actors can mixin with DiagnosticActorLogging which defines a diagnostic logger "log", has a hook to override for defining MDC values per message, and overrides aroundReceive for setting and clearing MDC around receive execution.
* Proper documentation was added for Scala and Java under the Logging/Slf4j section.

akka-docs/rst/java/code/docs/event/LoggingDocTest.java

@@ -22,6 +22,13 @@ import org.junit.Test;
 import akka.testkit.JavaTestKit;
 import scala.Option;
 
+//#imports-mdc
+import akka.event.Logging;
+import akka.event.DiagnosticLoggingAdapter;
+import java.util.HashMap;
+import java.util.Map;
+//#imports-mdc
+
 //#imports-deadletter
 import akka.actor.Props;
 import akka.actor.ActorRef;
@@ -40,6 +47,14 @@ public class LoggingDocTest {
     JavaTestKit.shutdownActorSystem(system);
   }
 
+  @Test
+  public void useLoggingActorWithMDC() {
+    ActorSystem system = ActorSystem.create("MyDiagnosticSystem");
+    ActorRef mdcActor = system.actorOf(Props.create(MdcActor.class, this));
+    mdcActor.tell("some request", ActorRef.noSender());
+    JavaTestKit.shutdownActorSystem(system);
+  }
+
   @Test
   public void subscribeToDeadLetters() {
     //#deadletters
@@ -86,6 +101,27 @@ public class LoggingDocTest {
 
   //#my-actor
 
+  //#mdc-actor
+  class MdcActor extends UntypedActor {
+
+      final DiagnosticLoggingAdapter log = Logging.getLogger(this);
+
+      public void onReceive(Object message) {
+
+          Map<String, Object> mdc;
+          mdc = new HashMap<String, Object>();
+          mdc.put("requestId", 1234);
+          mdc.put("visitorId", 5678);
+          log.setMDC(mdc);
+
+          log.info("Starting new request");
+
+          log.clearMDC();
+      }
+  }
+
+  //#mdc-actor
+
   //#my-event-listener
   class MyEventListener extends UntypedActor {
     public void onReceive(Object message) {