measure.go
func (m *MeasuredReader) BytesPer(perPeriod time.Duration) uint64 {
	return uint64(m.rate.Rate(perPeriod))
}
func (m *MeasuredReader) BytesPer(perPeriod time.Duration) uint64 {
	return uint64(m.rate.Rate(perPeriod))
}
// BytesPer tells the rate per period at which bytes were read since last
// measurement.