code: private synchronized void sendHeartBeat(
        final EvaluatorRuntimeProtocol.EvaluatorHeartbeatProto heartbeatProto) {
      if (LOG.isLoggable(Level.FINEST)) {
        LOG.log(Level.FINEST, "Heartbeat message:\n" + heartbeatProto, new Exception("Stack trace"));
      }
      this.evaluatorHeartbeatHandler.onNext(heartbeatProto);
    }
docs: Sends the actual heartbeat out and logs it, if so desired.
  
  @param heartbeatProto