[v9.4.x] Alerting: hide "silence" button for external AM setups (#62691)

Co-authored-by: Gilles De Mey <gilles.de.mey@gmail.com>

pkg/services/ngalert/api/api_configuration.go

@@ -148,8 +148,15 @@ func (srv ConfigSrv) RouteGetAlertingStatus(c *contextmodel.ReqContext) response
 		sendsAlertsTo = cfg.SendAlertsTo
 	}
 
+	// handle errors
+	externalAlertManagers, err := srv.externalAlertmanagers(c.Req.Context(), c.OrgID)
+	if err != nil {
+		return ErrResp(http.StatusInternalServerError, err, "")
+	}
+
 	resp := apimodels.AlertingStatus{
-		AlertmanagersChoice: apimodels.AlertmanagersChoice(sendsAlertsTo.String()),
+		AlertmanagersChoice:      apimodels.AlertmanagersChoice(sendsAlertsTo.String()),
+		NumExternalAlertmanagers: len(externalAlertManagers),
 	}
 	return response.JSON(http.StatusOK, resp)
 }

pkg/services/ngalert/api/tooling/definitions/admin.go

@@ -92,5 +92,6 @@ type GettableAlertmanagers struct {
 
 // swagger:model
 type AlertingStatus struct {
-	AlertmanagersChoice AlertmanagersChoice `json:"alertmanagersChoice"`
+	AlertmanagersChoice      AlertmanagersChoice `json:"alertmanagersChoice"`
+	NumExternalAlertmanagers int                 `json:"numExternalAlertmanagers"`
 }