Add fmt rule check: labels with the name __name__ are invalid (#7787)

Signed-off-by: Max Neverov <neverov.max@gmail.com>

pkg/rulefmt/rulefmt.go

@@ -186,7 +186,7 @@ func (r *RuleNode) Validate() (nodes []WrappedError) {
 	}
 
 	for k, v := range r.Labels {
-		if !model.LabelName(k).IsValid() {
+		if !model.LabelName(k).IsValid() || k == model.MetricNameLabel {
 			nodes = append(nodes, WrappedError{
 				err: errors.Errorf("invalid label name: %s", k),
 			})

pkg/rulefmt/rulefmt_test.go

@@ -71,6 +71,10 @@ func TestParseFileFailure(t *testing.T) {
 			filename: "bad_field.bad.yaml",
 			errMsg:   "field annotation not found",
 		},
+		{
+			filename: "invalid_label_name.bad.yaml",
+			errMsg:   "invalid label name",
+		},
 	}
 
 	for _, c := range table {

pkg/rulefmt/testdata/invalid_label_name.bad.yaml

@@ -0,0 +1,7 @@
+groups:
+- name: yolo
+  rules:
+  - record: hola
+    expr: 1
+    labels:
+      __name__: anything