code: func (h *headIndexReader) Series(ref uint64, lbls *labels.Labels, chks *[]chunks.Meta) error {
  	s := h.head.series.getByID(ref)
  
  	if s == nil {
  		h.head.metrics.seriesNotFound.Inc()
  		return ErrNotFound
  	}
  	*lbls = append((*lbls)[:0], s.lset...)
  
  	s.Lock()
  	defer s.Unlock()
  
  	*chks = (*chks)[:0]
  
  	for i, c := range s.chunks {
  		// Do not expose chunks that are outside of the specified range.
  		if !c.OverlapsClosedInterval(h.mint, h.maxt) {
  			continue
  		}
  		*chks = append(*chks, chunks.Meta{
  			MinTime: c.minTime,
  			MaxTime: c.maxTime,
  			Ref:     packChunkID(s.ref, uint64(s.chunkID(i))),
  		})
  	}
  
  	return nil
  }
docs: // Series returns the series for the given reference.