code: func (c *Counter) SetFreq(prevCount Counter, period time.Duration) {
  	if period == 0 {
  		return
  	}
  	freq := float64(c.Count-prevCount.Count) / float64(period/time.Second)
  	c.Freq = &freq
  }
docs: // SetFreq sets counter frequency based on the previous value
  // and the time period