instruments.go
func NewDeriveScale(v float64, d time.Duration) *Derive {
	return &Derive{
		value: math.Float64bits(v),
		rate: Rate{
			time: time.Now().UnixNano(),
			unit: d.Seconds(),
		},
	}
}
func NewDeriveScale(v float64, d time.Duration) *Derive {
	return &Derive{
		value: math.Float64bits(v),
		rate: Rate{
			time: time.Now().UnixNano(),
			unit: d.Seconds(),
		},
	}
}
// NewDeriveScale creates a new derive instruments with the given unit.