example for Streams log operator (#25283)

* and added all log levels in Attributes for convenient single access point

akka-docs/src/test/java/jdocs/stream/operators/SourceOrFlow.java

@@ -0,0 +1,27 @@
+/**
+ * Copyright (C) 2018 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package jdocs.stream.operators;
+
+import akka.stream.javadsl.Flow;
+
+//#log
+import akka.stream.Attributes;
+
+//#log
+
+class SourceOrFlow {
+
+  void logExample() {
+    Flow.of(String.class)
+      //#log
+      .log("myStream")
+      .addAttributes(Attributes.createLogLevels(
+        Attributes.logLevelOff(), // onElement
+        Attributes.logLevelError(), // onFailure
+        Attributes.logLevelInfo())) // onFinish
+    //#log
+    ;
+  }
+}