+str #18793 StageLogging that allows logger access in stages (#21696)

* +str #18793 StageLogging that allows logger access in stages
Also, non ActorMaterializers can opt-into providing a logger here.

* +str #18794 add javadsl for StageLogging

* fix missing test method on compile only class

akka-stream/src/main/scala/akka/stream/MaterializerLoggingProvider.scala

@@ -0,0 +1,17 @@
+/*
+ * Copyright (C) 2016 Lightbend Inc. <http://www.lightbend.com>
+ */
+
+package akka.stream
+
+import akka.event.LoggingAdapter
+
+/**
+ * SPI intended only to be extended by custom [[Materializer]] implementations,
+ * that also want to provide stages they materialize with specialized [[akka.event.LoggingAdapter]] instances.
+ */
+trait MaterializerLoggingProvider { this: Materializer ⇒
+
+  def makeLogger(logSource: Class[_]): LoggingAdapter
+
+}