code: public synchronized ComponentMetricStore getSubtaskMetricStore(String jobID, String taskID, int subtaskIndex) {
  		JobMetricStore job = jobID == null ? null : jobs.get(jobID);
  		if (job == null) {
  			return null;
  		}
  		TaskMetricStore task = job.getTaskMetricStore(taskID);
  		if (task == null) {
  			return null;
  		}
  		return ComponentMetricStore.unmodifiable(task.getSubtaskMetricStore(subtaskIndex));
  	}
docs: Returns the {@link ComponentMetricStore} for the given job/task ID and subtask index.
  
  @param jobID        job ID
  @param taskID       task ID
  @param subtaskIndex subtask index
  @return SubtaskMetricStore for the given IDs and index, or null if no store for the given arguments exists