code: func (r *Registry) RateScale(name string, tags []string, d time.Duration) *Rate {
  	factory := func() interface{} { return NewRateScale(d) }
  	return r.fetchRate(name, tags, factory)
  }
docs: // RateScale fetches an instrument from the registry or creates a new one
  // with a custom scale.
  //
  // If another instrument type is already registered with the same
  // name/tags, a blank one will be returned and an error
  // will be logged to the Errors() channel.