+str #15093 Add flow logging facility

* Fixes #15093
* Add Transformer.onError

akka-stream/src/main/scala/akka/stream/impl/ActorBasedFlowMaterializer.scala

@@ -142,6 +142,9 @@ private[akka] class ActorBasedFlowMaterializer(
     case x    ⇒ x
   }
 
+  def withNamePrefix(name: String): FlowMaterializer =
+    new ActorBasedFlowMaterializer(settings, _context, name)
+
   private def system: ActorSystem = _context match {
     case s: ExtendedActorSystem ⇒ s
     case c: ActorContext        ⇒ c.system
@@ -151,9 +154,6 @@ private[akka] class ActorBasedFlowMaterializer(
 
   private def nextFlowNameCount(): Long = FlowNameCounter(system).counter.incrementAndGet()
 
-  def withNamePrefix(name: String): FlowMaterializer =
-    new ActorBasedFlowMaterializer(settings, _context, name)
-
   private def createFlowName(): String = s"$namePrefix-${nextFlowNameCount()}"
 
   @tailrec private def processorChain(topConsumer: Consumer[_], ops: immutable.Seq[AstNode],