code: func Iter(cb func(m types.Metric)) {
  	lock.RLock()
  	defer lock.RUnlock()
  	for _, v := range registry {
  		cb(v)
  	}
  }
docs: // Iter calls a callback for each registered metric.
  //
  // Metrics are visited in no particular order. The callback must not modify
  // the registry.