convenience.go
func (r *Registry) Gauge(name string, tags []string) *Gauge {
	return r.fetchGauge(name, tags, newGauge)
}
func (r *Registry) Gauge(name string, tags []string) *Gauge {
	return r.fetchGauge(name, tags, newGauge)
}
// Gauge 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.