convenience.go
func (r *Registry) Rate(name string, tags []string) *Rate {
	return r.fetchRate(name, tags, newRate)
}
func (r *Registry) Rate(name string, tags []string) *Rate {
	return r.fetchRate(name, tags, newRate)
}
// Rate fetches an instrument from the registry or creates a new one.
//
// 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.