registry.go
func (r *Registry) Size() int {
	r.mutex.RLock()
	size := len(r.instruments)
	r.mutex.RUnlock()
	return size
}
func (r *Registry) Size() int {
	r.mutex.RLock()
	size := len(r.instruments)
	r.mutex.RUnlock()
	return size
}
// Size returns the numbers of instruments in the registry.