title: Akka: Get total number of actors
body: <p>Is there an easy way to get the total number of all living Actors of an ActorSystem (with the Java8 API)?</p>
  
  <p>It would be useful for me for debugging my application: I create Actors in many different places and want to be sure that they all terminate correctly. An actor children counter would help me to see if the total number goes up or stays approximately the same when I run my application over weeks.</p>