akka_setup.scala

implicit val system = ActorSystem("QuickStart")
implicit val materializer = ActorMaterializer()

def peekMatValue[T, M](src: Source[T, M]): (Source[T, M], Future[M]) = {
  val p = Promise[M]
  val s = src.mapMaterializedValue { m =>
    p.trySuccess(m)
    m
  }
  (s, p.future)
}

val (bloodPressureSource, bloodPressureQueueFuture) = peekMatValue(Source.queue[String](100, OverflowStrategy.dropHead))
val (heartRateSource, heartRateQueueFuture) = peekMatValue(Source.queue[String](100, OverflowStrategy.dropHead))