Alerting: Implement /status for the notification system (#33227)

* Alerting: Implement /status for the notification system

Implements the necessary plumbing to have a /status endpoint on the
notification system.

* Add API examples

* Update API specs

* Update prometheus/common dependency

Co-authored-by: Sofia Papagiannaki <sofia@grafana.com>

pkg/services/ngalert/api/api_alertmanager.go

@@ -21,6 +21,10 @@ type AlertmanagerSrv struct {
 	log   log.Logger
 }
 
+func (srv AlertmanagerSrv) RouteGetAMStatus(c *models.ReqContext) response.Response {
+	return response.JSON(http.StatusOK, srv.am.GetStatus())
+}
+
 func (srv AlertmanagerSrv) RouteCreateSilence(c *models.ReqContext, postableSilence apimodels.PostableSilence) response.Response {
 	if !c.HasUserRole(models.ROLE_EDITOR) {
 		return ErrResp(http.StatusForbidden, errors.New("permission denied"), "")