code: func (q externalLabelsQuerier) Select(p *storage.SelectParams, matchers ...*labels.Matcher) (storage.SeriesSet, storage.Warnings, error) {
  	m, added := q.addExternalLabels(matchers)
  	s, warnings, err := q.Querier.Select(p, m...)
  	if err != nil {
  		return nil, warnings, err
  	}
  	return newSeriesSetFilter(s, added), warnings, nil
  }
docs: // Select adds equality matchers for all external labels to the list of matchers
  // before calling the wrapped storage.Queryable. The added external labels are
  // removed from the returned series sets.