code: func PreferLocalStorageFilter(next storage.Queryable, cb startTimeCallback) storage.Queryable {
  	return storage.QueryableFunc(func(ctx context.Context, mint, maxt int64) (storage.Querier, error) {
  		localStartTime, err := cb()
  		if err != nil {
  			return nil, err
  		}
  		cmaxt := maxt
  		// Avoid queries whose timerange is later than the first timestamp in local DB.
  		if mint > localStartTime {
  			return storage.NoopQuerier(), nil
  		}
  		// Query only samples older than the first timestamp in local DB.
  		if maxt > localStartTime {
  			cmaxt = localStartTime
  		}
  		return next.Querier(ctx, mint, cmaxt)
  	})
  }
docs: // PreferLocalStorageFilter returns a QueryableFunc which creates a NoopQuerier
  // if requested timeframe can be answered completely by the local TSDB, and
  // reduces maxt if the timeframe can be partially answered by TSDB.