code: func (p *MemPostings) Add(id uint64, lset labels.Labels) {
  	p.mtx.Lock()
  
  	for _, l := range lset {
  		p.addFor(id, l)
  	}
  	p.addFor(id, allPostingsKey)
  
  	p.mtx.Unlock()
  }
docs: // Add a label set to the postings index.