message: Prevent `KeyError` by having default health as UNKNOWN
diff: @@ -86,7 +86,8 @@ class _TPUPollingThread(threading.Thread):
  response = self._cluster._fetch_cloud_tpu_metadata() # pylint: disable=protected-access
  logging.warning(
  'TPUPollingThread found TPU %s in state %s, and health %s.',
  - self._cluster._tpu, response['state'], response['health']) # pylint: disable=protected-access
  + self._cluster._tpu, response['state'], # pylint: disable=protected-access
  + response.get('health', 'UNKNOWN'))
  if 'state' in response and response['state'] in [
  'TERMINATED', 'PREEMPTED'
  