code: func (p *Partition) TagKeyIterator(name []byte) tsdb.TagKeyIterator {
  	fs, err := p.RetainFileSet()
  	if err != nil {
  		return nil // TODO(edd): this should probably return an error.
  	}
  
  	itr := fs.TagKeyIterator(name)
  	if itr == nil {
  		fs.Release()
  		return nil
  	}
  	return newFileSetTagKeyIterator(fs, NewTSDBTagKeyIteratorAdapter(itr))
  }
docs: // TagKeyIterator returns an iterator for all keys across a single measurement.