Make sure static mdc from withMdc set during actor startup (#29894)

* Make sure static mdc from withMdc set during actor startup #29893

* Actually assert mdc is included

akka-actor-typed-tests/src/test/scala/akka/actor/typed/scaladsl/ActorLoggingSpec.scala

@@ -329,7 +329,7 @@ class ActorLoggingSpec extends ScalaTestWithActorTestKit("""
     "provide the MDC values in the log" in {
       val behaviors = Behaviors.withMdc[Protocol](
         Map("static" -> "1"),
-        // FIXME why u no infer the type here Scala??
+        // why u no infer the type here Scala??
         (message: Protocol) =>
           if (message.transactionId == 1)
             Map("txId" -> message.transactionId.toString, "first" -> "true")
@@ -348,7 +348,9 @@ class ActorLoggingSpec extends ScalaTestWithActorTestKit("""
         .info("Starting")
         // not counting for example "akkaSource", but it shouldn't have any other entries
         .withCustom(logEvent =>
-          logEvent.mdc.keysIterator.forall(entry => entry.startsWith("akka") || entry == "sourceActorSystem"))
+          logEvent.mdc.keysIterator.forall(entry =>
+            entry.startsWith("akka") || entry == "sourceActorSystem" || entry == "static") &&
+          logEvent.mdc("static") == "1")
         .expect {
           spawn(behaviors)
         }

akka-actor-typed/src/main/scala/akka/actor/typed/internal/WithMdcBehaviorInterceptor.scala

@@ -62,8 +62,12 @@ import akka.annotation.InternalApi
         case b => b
       }
     }
-
-    loop(target.start(ctx))
+    try {
+      setMdcValues(Map.empty)
+      loop(target.start(ctx))
+    } finally {
+      MDC.clear()
+    }
   }
 
   // in the normal case, a new withMDC replaces the previous one