LibraryElements: Creates usage stats for panels and variables (#34476) (#34555)

* LibraryPanels: Adds usage collection

* Refactor: renames Panel and Variable consts

* Chore: initialize stats

* Refactor: moves library element migrations to migration namespace

(cherry picked from commit 7204a64717)

Co-authored-by: Hugo Häggmark <hugo.haggmark@grafana.com>

pkg/services/libraryelements/libraryelements_get_test.go

@@ -35,7 +35,7 @@ func TestGetLibraryElement(t *testing.T) {
 						FolderID:    1,
 						UID:         res.Result.UID,
 						Name:        "Text - Library Panel",
-						Kind:        int64(Panel),
+						Kind:        int64(models.PanelElement),
 						Type:        "text",
 						Description: "A description",
 						Model: map[string]interface{}{
@@ -130,7 +130,7 @@ func TestGetLibraryElement(t *testing.T) {
 						FolderID:    1,
 						UID:         res.Result.UID,
 						Name:        "Text - Library Panel",
-						Kind:        int64(Panel),
+						Kind:        int64(models.PanelElement),
 						Type:        "text",
 						Description: "A description",
 						Model: map[string]interface{}{