promql/engine.go
func signatureFunc(on bool, names ...string) func(labels.Labels) uint64 {
	// TODO(fabxc): ensure names are sorted and then use that and sortedness
	// of labels by names to speed up the operations below.
	// Alternatively, inline the hashing and don't build new label sets.
	if on {
		return func(lset labels.Labels) uint64 { return lset.HashForLabels(names...) }
	}
	return func(lset labels.Labels) uint64 { return lset.HashWithoutLabels(names...) }
}
func signatureFunc(on bool, names ...string) func(labels.Labels) uint64 {
	// TODO(fabxc): ensure names are sorted and then use that and sortedness
	// of labels by names to speed up the operations below.
	// Alternatively, inline the hashing and don't build new label sets.
	if on {
		return func(lset labels.Labels) uint64 { return lset.HashForLabels(names...) }
	}
	return func(lset labels.Labels) uint64 { return lset.HashWithoutLabels(names...) }
}
// signatureFunc returns a function that calculates the signature for a metric
// ignoring the provided labels. If on, then the given labels are only used instead.