measure.go
func (m *MeasuredWriter) BytesPerSec() uint64 {
	return uint64(m.rate.Rate(time.Second))
}
func (m *MeasuredWriter) BytesPerSec() uint64 {
	return uint64(m.rate.Rate(time.Second))
}
// BytesPerSec tells the rate per second at which bytes were written since last
// measurement.