registry.go
func (r *Registry) Get(name string, tags []string) interface{} {
	key := MetricID(name, tags)
	r.mutex.RLock()
	v := r.instruments[key]
	r.mutex.RUnlock()
	return v
}
func (r *Registry) Get(name string, tags []string) interface{} {
	key := MetricID(name, tags)
	r.mutex.RLock()
	v := r.instruments[key]
	r.mutex.RUnlock()
	return v
}
// Get returns an instrument from the Registry.