code: 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...) }
  }
docs: // 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.