code: func (s *summary) flushColdBuf() {
  	for _, v := range s.coldBuf {
  		for _, stream := range s.streams {
  			stream.Insert(v)
  		}
  		s.cnt++
  		s.sum += v
  	}
  	s.coldBuf = s.coldBuf[0:0]
  	s.maybeRotateStreams()
  }
docs: // flushColdBuf needs mtx locked.