docs: def wait_for_instance_deletion(credentials, project, zone, instance_name,
                                 interval_seconds=5):
      """Wait until an instance is deleted.
      
      We require that initially, the specified instance exists.
      TODO: docstring
      """
      
      t0 = time.time()
      access_token = credentials.get_access_token()
      headers = {
          'Authorization': 'Bearer %s' % access_token.access_token
      }
  
      r = requests.get('https://www.googleapis.com/compute/v1/'
                       'projects/%s/zones/%s/instances/%s'
                       % (project, zone, instance_name),
                       headers=headers)
      
      if r.status_code == 404:
          raise AssertionError('Instance "%s" does not exist!' % instance_name)
          
      r.raise_for_status()
      _LOGGER.debug('Instance "%s" exists.', instance_name)
  
      while True:
          time.sleep(interval_seconds)
  
          access_token = credentials.get_access_token()
          headers = {
              'Authorization': 'Bearer %s' % access_token.access_token
          }
  
          r = requests.get('https://www.googleapis.com/compute/v1/'
                           'projects/%s/zones/%s/instances/%s'
                           % (project, zone, instance_name),
                           headers=headers)
          if r.status_code == 404:
              break
          r.raise_for_status()
          _LOGGER.debug('Instance "%s" still exists.', instance_name)
          
      t1 = time.time()
      t = t1-t0
      t_min = t/60.0
      _LOGGER.info('Instance was deleted after %.1f s (%.1f m).', t, t_min)