code: func (dc *Config) WithHandler(c context.Context, l caching.Loader, timeout time.Duration) context.Context {
  	handler := dsCacheHandler{
  		refreshInterval: dc.RefreshInterval,
  		failOpen:        dc.FailOpen,
  		lockerFunc:      dc.LockerFunc,
  		loader:          l,
  		loaderTimeout:   timeout,
  	}
  	return context.WithValue(c, &dsHandlerKey, &handler)
  }
docs: // WithHandler installs a datastorecache.Handler into our Context. This is
  // used during datastore cache's Refresh calls.
  //
  // The Handler binds the parameters and Loader to the resolution call. For
  // service resolution, this will be the Loader that is provided by the caching
  // layer. For cron refresh, this will be the generic Loader provided by
  // CronLoader.