514 - bring back mailbox size metric

[skipper1982]

Closes #514

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Summary:

(please input the summary)

[lgajowy]

@skipper1982 I think I have an idea on how to use an async instrument for this metric (alternative approach). LMKWDYT. :)

[lgajowy]

There's a method in ActorCell:

 /**
   * If the actor isLocal, returns the number of "user messages" currently queued,
   * which may be a costly operation, 0 otherwise.
   */
  def numberOfMessages: Int

We could (maybe?) pass a reference to that field every time an actor is created.

object LocalActorRefProviderAdvice {

  @OnMethodExit
  def actorOf(@Return ref: ActorRef, @Argument(0) system: ActorSystem): Unit =
    ??? // get the reference, the path and create the instrument for the actorRef (that contains the actorcell, which has the numberOfMessages method).

}

that advice could be attached to:

private val localActorRefProviderInstrumentation: AgentInstrumentation =
    instrument("akka.actor.LocalActorRefProvider").visit(LocalActorRefProviderAdvice, "actorOf")

(taken from v0.7.0) Not sure if it's enough to instrument the localActorRefProvider

everytime we create a new actor maybe we can setup an async instrument that would periodically poll for the value from numberOfMessages, thus taking the mailbox size. Gauge seems a good fit. Not sure about the performance of this (probably awful, but maybe not?).

@skipper1982 lmk what do you think. I realize this can be unclear so let's have a call if needed. :)

[lgajowy]

@adamgadomski I created an issue for a potential improvement of this metric: #615

For now merging as is.