code: func (h *Histogram) Merge(from *Histogram) (dropped int64) {
  	i := from.rIterator()
  	for i.next() {
  		v := i.valueFromIdx
  		c := i.countAtIdx
  
  		if h.RecordValues(v, c) != nil {
  			dropped += c
  		}
  	}
  
  	return
  }
docs: // Merge merges the data stored in the given histogram with the receiver,
  // returning the number of recorded values which had to be dropped.