convenience.go
func (r *Registry) Timer(name string, tags []string) *Timer {
	factory := func() interface{} { return NewTimer() }
	return r.fetchTimer(name, tags, factory)
}
func (r *Registry) Timer(name string, tags []string) *Timer {
	factory := func() interface{} { return NewTimer() }
	return r.fetchTimer(name, tags, factory)
}
// Timer 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.