code: def _get_authorization_headers(self, context):
          
          headers = {}
          self._credentials.before_request(
              self._request,
              context.method_name,
              context.service_url,
              headers)
  
          return list(six.iteritems(headers))
docs: Gets the authorization headers for a request.
  
          Returns:
              Sequence[Tuple[str, str]]: A list of request headers (key, value)
                  to add to the request.