code: func (q requiredMatchersQuerier) Select(p *storage.SelectParams, matchers ...*labels.Matcher) (storage.SeriesSet, storage.Warnings, error) {
  	ms := q.requiredMatchers
  	for _, m := range matchers {
  		for i, r := range ms {
  			if m.Type == labels.MatchEqual && m.Name == r.Name && m.Value == r.Value {
  				ms = append(ms[:i], ms[i+1:]...)
  				break
  			}
  		}
  		if len(ms) == 0 {
  			break
  		}
  	}
  	if len(ms) > 0 {
  		return storage.NoopSeriesSet(), nil, nil
  	}
  	return q.Querier.Select(p, matchers...)
  }
docs: // Select returns a NoopSeriesSet if the given matchers don't match the label
  // set of the requiredMatchersQuerier. Otherwise it'll call the wrapped querier.