code: def add_response(self, req, resp):
          
          if self._cache is None:
              return
          signature = sign(req.allocateQuotaRequest)
          with self._cache as c:
              now = self._timer()
              item = c.get(signature)
              if item is None:
                  c[signature] = CachedItem(
                      req, resp, self.service_name, now)
              else:
                  # Update the cached item to reflect that it is updated
                  item.last_check_time = now
                  item.response = resp
                  item.is_in_flight = False
                  c[signature] = item
docs: Adds the response from sending to `req` to this instance's cache.
  
          Args:
            req (`ServicecontrolServicesAllocateQuotaRequest`): the request
            resp (AllocateQuotaResponse): the response from sending the request