Start the thread that registers the size of the DataStore.

def InitializeMonitorThread(self):
    """Start the thread that registers the size of the DataStore."""
    if self.monitor_thread:
      return
    self.monitor_thread = utils.InterruptableThread(
        name="DataStore monitoring thread",
        target=self._RegisterSize,
        sleep_time=60)
    self.monitor_thread.start()