instruments.go
func NewRateScale(d time.Duration) *Rate {
	return &Rate{
		time: time.Now().UnixNano(),
		unit: d.Seconds(),
	}
}
func NewRateScale(d time.Duration) *Rate {
	return &Rate{
		time: time.Now().UnixNano(),
		unit: d.Seconds(),
	}
}
// NewRateScale creates a new rate instruments with the given unit.