code: func SortByItemToPB(sc *stmtctx.StatementContext, client kv.Client, expr Expression, desc bool) *tipb.ByItem {
  	pc := PbConverter{client: client, sc: sc}
  	e := pc.ExprToPB(expr)
  	if e == nil {
  		return nil
  	}
  	return &tipb.ByItem{Expr: e, Desc: desc}
  }
docs: // SortByItemToPB converts order by items to pb.