convenience.go
func (r *Registry) RateScale(name string, tags []string, d time.Duration) *Rate {
	factory := func() interface{} { return NewRateScale(d) }
	return r.fetchRate(name, tags, factory)
}
func (r *Registry) RateScale(name string, tags []string, d time.Duration) *Rate {
	factory := func() interface{} { return NewRateScale(d) }
	return r.fetchRate(name, tags, factory)
}
// 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.