code: def _do_refresh_request(self, unused_http_request):
          
          relative_url = 'instance/service-accounts/{0}/token'.format(
              self.__service_account_name)
          try:
              response = _GceMetadataRequest(relative_url)
          except exceptions.CommunicationError:
              self.invalid = True
              if self.store:
                  self.store.locked_put(self)
              raise
          content = response.read()
          try:
              credential_info = json.loads(content)
          except ValueError:
              raise exceptions.CredentialsError(
                  'Could not parse response as JSON: %s' % content)
  
          self.access_token = credential_info['access_token']
          if 'expires_in' in credential_info:
              expires_in = int(credential_info['expires_in'])
              self.token_expiry = (
                  datetime.timedelta(seconds=expires_in) +
                  datetime.datetime.utcnow())
          else:
              self.token_expiry = None
          self.invalid = False
          if self.store:
              self.store.locked_put(self)
docs: Refresh self.access_token by querying the metadata server.
  
          If self.store is initialized, store acquired credentials there.