class_docstr: Monitor a given trial inside a thread, updating its heartbeat
  at a given interval of time.
  
  Parameters
  ----------
  exp: Experiment
      The current Experiment.
code: def _monitor_trial(self):
      trial = self.storage.get_trial(self.trial)
  
      if trial.status in STOPPED_STATUS:
          self.stopped.set()
      else:
          if not self.storage.update_heartbeat(trial):
              self.stopped.set()
  
imports: import threading
label_desc: Use image node_id 4 for calling the TrialPacemaker obj's underlying member method code with example usage: obj._monitor_trial() without return types