code: func (r *Registry) Register(name string, tags []string, v interface{}) {
  	switch v.(type) {
  	case Discrete, Sample:
  		key := MetricID(name, tags)
  		r.mutex.Lock()
  		r.instruments[key] = v
  		r.mutex.Unlock()
  	}
  }
docs: // Register registers a new instrument.