storage/fanout.go
func (q *mergeQuerier) LabelValues(name string) ([]string, error) {
	var results [][]string
	for _, querier := range q.queriers {
		values, err := querier.LabelValues(name)
		if err != nil {
			return nil, err
		}
		results = append(results, values)
	}
	return mergeStringSlices(results), nil
}
func (q *mergeQuerier) LabelValues(name string) ([]string, error) {
	var results [][]string
	for _, querier := range q.queriers {
		values, err := querier.LabelValues(name)
		if err != nil {
			return nil, err
		}
		results = append(results, values)
	}
	return mergeStringSlices(results), nil
}
// LabelValues returns all potential values for a label name.