convenience.go
func (r *Registry) Derive(name string, tags []string, v float64) *Derive {
	factory := func() interface{} { return NewDerive(v) }
	return r.fetchDerive(name, tags, factory)
}
func (r *Registry) Derive(name string, tags []string, v float64) *Derive {
	factory := func() interface{} { return NewDerive(v) }
	return r.fetchDerive(name, tags, factory)
}
// Derive 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.