code: def add_data_list(self, dl):  # pragma: no cover - never used...
          
          if self.__data_lock.acquire():
              self.__data.extend(dl)
              self.__data_lock.release()
              return True
          return False
docs: dl must be a list of tuples like:
          dl = [('metricname', (timestamp, value)),
                ('metricname', (timestamp, value)),
                ...]