code: func (h *Histogram) StdDev() float64 {
  	if h.totalCount == 0 {
  		return 0
  	}
  
  	mean := h.Mean()
  	geometricDevTotal := 0.0
  
  	i := h.iterator()
  	for i.next() {
  		if i.countAtIdx != 0 {
  			dev := float64(h.medianEquivalentValue(i.valueFromIdx)) - mean
  			geometricDevTotal += (dev * dev) * float64(i.countAtIdx)
  		}
  	}
  
  	return math.Sqrt(geometricDevTotal / float64(h.totalCount))
  }
docs: // StdDev returns the approximate standard deviation of the recorded values.