code: func (p *Partition) TagKeySeriesIDIterator(name, key []byte) tsdb.SeriesIDIterator {
  	fs, err := p.RetainFileSet()
  	if err != nil {
  		return nil // TODO(edd): this should probably return an error.
  	}
  
  	itr := fs.TagKeySeriesIDIterator(name, key)
  	if itr == nil {
  		fs.Release()
  		return nil
  	}
  	return newFileSetSeriesIDIterator(fs, itr)
  }
docs: // TagKeySeriesIDIterator returns a series iterator for all values across a single key.