Ensure series in matrix values returned for instant queries are alway…

…s sorted

Signed-off-by: Charles Korn <charles.korn@grafana.com>

promql/engine.go

@@ -752,6 +752,7 @@ func (ng *Engine) execEvalStmt(ctx context.Context, query *query, s *parser.Eval
 		case parser.ValueTypeScalar:
 			return Scalar{V: mat[0].Floats[0].F, T: start}, warnings, nil
 		case parser.ValueTypeMatrix:
+			sort.Sort(mat)
 			return mat, warnings, nil
 		default:
 			panic(fmt.Errorf("promql.Engine.exec: unexpected expression type %q", s.Expr.Type()))