code: func (sg *SampleGroup) StdDev() float64 {
  	avg := float64(sg.AvgRate())
  	sum := float64(0)
  	for _, c := range sg.Samples {
  		sum += math.Pow(float64(c.Rate())-avg, 2)
  	}
  	variance := sum / float64(len(sg.Samples))
  	return math.Sqrt(variance)
  }
docs: // StdDev returns the standard deviation the message rates in the SampleGroup