code: func (p *Partition) ForEachMeasurementTagKey(name []byte, fn func(key []byte) error) error {
  	fs, err := p.RetainFileSet()
  	if err != nil {
  		return err
  	}
  	defer fs.Release()
  
  	itr := fs.TagKeyIterator(name)
  	if itr == nil {
  		return nil
  	}
  
  	for e := itr.Next(); e != nil; e = itr.Next() {
  		if err := fn(e.Key()); err != nil {
  			return err
  		}
  	}
  
  	return nil
  }
docs: // ForEachMeasurementTagKey iterates over all tag keys in a measurement.