promql/engine.go
func (enh *EvalNodeHelper) dropMetricName(l labels.Labels) labels.Labels {
	if enh.dmn == nil {
		enh.dmn = make(map[uint64]labels.Labels, len(enh.out))
	}
	h := l.Hash()
	ret, ok := enh.dmn[h]
	if ok {
		return ret
	}
	ret = dropMetricName(l)
	enh.dmn[h] = ret
	return ret
}
func (enh *EvalNodeHelper) dropMetricName(l labels.Labels) labels.Labels {
	if enh.dmn == nil {
		enh.dmn = make(map[uint64]labels.Labels, len(enh.out))
	}
	h := l.Hash()
	ret, ok := enh.dmn[h]
	if ok {
		return ret
	}
	ret = dropMetricName(l)
	enh.dmn[h] = ret
	return ret
}
// dropMetricName is a cached version of dropMetricName.