code: func RequiredMatchersFilter(next storage.Queryable, required []*labels.Matcher) storage.Queryable {
  	return storage.QueryableFunc(func(ctx context.Context, mint, maxt int64) (storage.Querier, error) {
  		q, err := next.Querier(ctx, mint, maxt)
  		if err != nil {
  			return nil, err
  		}
  		return &requiredMatchersQuerier{Querier: q, requiredMatchers: required}, nil
  	})
  }
docs: // RequiredMatchersFilter returns a storage.Queryable which creates a
  // requiredMatchersQuerier.