convenience.go
func (r *Registry) fetchCounter(name string, tags []string, factory func() interface{}) *Counter {
	v := r.Fetch(name, tags, factory)
	if i, ok := v.(*Counter); ok {
		return i
	}
	r.handleFetchError("counter", name, tags, v)
	return factory().(*Counter)
}
func (r *Registry) fetchCounter(name string, tags []string, factory func() interface{}) *Counter {
	v := r.Fetch(name, tags, factory)
	if i, ok := v.(*Counter); ok {
		return i
	}
	r.handleFetchError("counter", name, tags, v)
	return factory().(*Counter)
}
// --------------------------------------------------------------------