code: def get_token(http, service_account='default'):
      
      token_json = get(
          http,
          'instance/service-accounts/{0}/token'.format(service_account))
      token_expiry = client._UTCNOW() + datetime.timedelta(
          seconds=token_json['expires_in'])
      return token_json['access_token'], token_expiry
docs: Fetch an oauth token for the
  
      Args:
          http: an object to be used to make HTTP requests.
          service_account: An email specifying the service account this token
              should represent. Default will be a token for the "default" service
              account of the current compute engine instance.
  
      Returns:
           A tuple of (access token, token expiration), where access token is the
           access token as a string and token expiration is a datetime object
           that indicates when the access token will expire.