feat: show query to not invoke reserve conn and show variables should be able to return the updated value

Signed-off-by: Harshit Gangal <harshit@planetscale.com>

go/vt/sysvars/sysvars.go

@@ -260,5 +260,11 @@ func GetInterestingVariables() []string {
 	res = append(res, Version.Name)
 	res = append(res, VersionComment.Name)
 	res = append(res, Socket.Name)
+
+	for _, variable := range UseReservedConn {
+		if variable.SupportSetVar {
+			res = append(res, variable.Name)
+		}
+	}
 	return res
 }

go/vt/vtgate/executor.go

@@ -1036,7 +1036,7 @@ func prepareSetVarComment(vcursor *vcursorImpl, stmt sqlparser.Statement) (strin
 	switch stmt.(type) {
 	// If the statement is a transaction statement or a set no reserved connection / SET_VAR is needed
 	case *sqlparser.Begin, *sqlparser.Commit, *sqlparser.Rollback, *sqlparser.Savepoint,
-		*sqlparser.SRollback, *sqlparser.Release, *sqlparser.Set:
+		*sqlparser.SRollback, *sqlparser.Release, *sqlparser.Set, *sqlparser.Show:
 		return "", nil
 	case sqlparser.SupportOptimizerHint:
 		break