refactor: check for variants before samples in roundtripper

pkg/querier/queryrange/roundtrip.go

@@ -416,6 +416,31 @@ func (r roundTripper) Do(ctx context.Context, req base.Request) (base.Response,
 		}
 
 		switch e := op.Plan.AST.(type) {
+		case syntax.VariantsExpr:
+			if err := validateMaxEntriesLimits(ctx, op.Limit, r.limits); err != nil {
+				return nil, httpgrpc.Errorf(http.StatusBadRequest, "%s", err.Error())
+			}
+
+			matchers := e.Matchers()
+
+			if err := validateMatchers(ctx, r.limits, matchers); err != nil {
+				return nil, httpgrpc.Errorf(http.StatusBadRequest, "%s", err.Error())
+			}
+
+			for _, v := range e.Variants() {
+				groups, err := v.MatcherGroups()
+				if err != nil {
+					level.Warn(logger).Log("msg", "unexpected matcher groups error in roundtripper", "err", err)
+				}
+
+				for _, g := range groups {
+					if err := validateMatchers(ctx, r.limits, g.Matchers); err != nil {
+						return nil, httpgrpc.Errorf(http.StatusBadRequest, "%s", err.Error())
+					}
+				}
+			}
+
+			return r.variants.Do(ctx, req)
 		case syntax.SampleExpr:
 			// The error will be handled later.
 			groups, err := e.MatcherGroups()
@@ -444,31 +469,6 @@ func (r roundTripper) Do(ctx context.Context, req base.Request) (base.Response,
 				return r.limited.Do(ctx, req)
 			}
 			return r.log.Do(ctx, req)
-		case syntax.VariantsExpr:
-			if err := validateMaxEntriesLimits(ctx, op.Limit, r.limits); err != nil {
-				return nil, httpgrpc.Errorf(http.StatusBadRequest, "%s", err.Error())
-			}
-
-			matchers := e.Matchers()
-
-			if err := validateMatchers(ctx, r.limits, matchers); err != nil {
-				return nil, httpgrpc.Errorf(http.StatusBadRequest, "%s", err.Error())
-			}
-
-			for _, v := range e.Variants() {
-				groups, err := v.MatcherGroups()
-				if err != nil {
-					level.Warn(logger).Log("msg", "unexpected matcher groups error in roundtripper", "err", err)
-				}
-
-				for _, g := range groups {
-					if err := validateMatchers(ctx, r.limits, g.Matchers); err != nil {
-						return nil, httpgrpc.Errorf(http.StatusBadRequest, "%s", err.Error())
-					}
-				}
-			}
-
-			return r.variants.Do(ctx, req)
 		default:
 			return r.next.Do(ctx, req)
 		}