code: func (p *Partition) MeasurementIterator() (tsdb.MeasurementIterator, error) {
  	fs, err := p.RetainFileSet()
  	if err != nil {
  		return nil, err
  	}
  	itr := fs.MeasurementIterator()
  	if itr == nil {
  		fs.Release()
  		return nil, nil
  	}
  	return newFileSetMeasurementIterator(fs, NewTSDBMeasurementIteratorAdapter(itr)), nil
  }
docs: // MeasurementIterator returns an iterator over all measurement names.