code: func (p *Partition) MeasurementTagKeysByExpr(name []byte, expr influxql.Expr) (map[string]struct{}, error) {
  	fs, err := p.RetainFileSet()
  	if err != nil {
  		return nil, err
  	}
  	defer fs.Release()
  
  	return fs.MeasurementTagKeysByExpr(name, expr)
  }
docs: // MeasurementTagKeysByExpr extracts the tag keys wanted by the expression.