registry.go
func (r *Registry) Unregister(name string, tags []string) {
	key := MetricID(name, tags)
	r.mutex.Lock()
	delete(r.instruments, key)
	r.mutex.Unlock()
}
func (r *Registry) Unregister(name string, tags []string) {
	key := MetricID(name, tags)
	r.mutex.Lock()
	delete(r.instruments, key)
	r.mutex.Unlock()
}
// Unregister remove from the registry the instrument matching the given name/tags