code: func (r *Registry) Rate(name string, tags []string) *Rate {
  	return r.fetchRate(name, tags, newRate)
  }
docs: // 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.