Alerting: Add first Grafana reserved label grafana_folder (#50262)

* Alerting: Add first Grafana reserved label g_label

g_label holds the title of the folder container the alert. The intention of this label
is to use it as part of the new default notification policy groupBy.

* Add nil check on updateRule labels map

* Disable gocyclo lint on schedule.ruleRoutine

will remove later in a separate refactoring PR to reduce complexity.

* Address doc suggestions

* Update g_folder for rules in folder when folder title changes

* Remove global bus in FolderService

* Modify tests to fit new common g_folder label

* Add changelog entry

* Fix merge conflicts

* Switch GrafanaReservedLabelPrefix from `g_` to `grafana_`

pkg/services/libraryelements/libraryelements_test.go

@@ -13,6 +13,7 @@ import (
 	"github.com/stretchr/testify/require"
 
 	"github.com/grafana/grafana/pkg/api/response"
+	busmock "github.com/grafana/grafana/pkg/bus/mock"
 	"github.com/grafana/grafana/pkg/components/simplejson"
 	"github.com/grafana/grafana/pkg/models"
 	acmock "github.com/grafana/grafana/pkg/services/accesscontrol/mock"
@@ -236,7 +237,7 @@ func createFolderWithACL(t *testing.T, sqlStore *sqlstore.SQLStore, title string
 	)
 	s := dashboardservice.ProvideFolderService(
 		cfg, d, dashboardStore, nil,
-		features, folderPermissions, ac,
+		features, folderPermissions, ac, busmock.New(),
 	)
 	t.Logf("Creating folder with title and UID %q", title)
 	folder, err := s.CreateFolder(context.Background(), &user, user.OrgId, title, title)
@@ -340,7 +341,7 @@ func testScenario(t *testing.T, desc string, fn func(t *testing.T, sc scenarioCo
 			SQLStore: sqlStore,
 			folderService: dashboardservice.ProvideFolderService(
 				cfg, dashboardService, dashboardStore, nil,
-				features, folderPermissions, ac,
+				features, folderPermissions, ac, busmock.New(),
 			),
 		}