Alerting: Feature toggle to disallow sending alerts externally (#87982)

* Define feature toggle

* Implement feature toggle

pkg/services/ngalert/api/api_configuration.go

@@ -12,6 +12,7 @@ import (
 	"github.com/grafana/grafana/pkg/infra/log"
 	contextmodel "github.com/grafana/grafana/pkg/services/contexthandler/model"
 	"github.com/grafana/grafana/pkg/services/datasources"
+	"github.com/grafana/grafana/pkg/services/featuremgmt"
 	apimodels "github.com/grafana/grafana/pkg/services/ngalert/api/tooling/definitions"
 	ngmodels "github.com/grafana/grafana/pkg/services/ngalert/models"
 	"github.com/grafana/grafana/pkg/services/ngalert/store"
@@ -24,6 +25,7 @@ type ConfigSrv struct {
 	alertmanagerProvider ExternalAlertmanagerProvider
 	store                store.AdminConfigurationStore
 	log                  log.Logger
+	featureManager       featuremgmt.FeatureToggles
 }
 
 func (srv ConfigSrv) RouteGetAlertmanagers(c *contextmodel.ReqContext) response.Response {
@@ -75,6 +77,11 @@ func (srv ConfigSrv) RoutePostNGalertConfig(c *contextmodel.ReqContext, body api
 		return response.Error(http.StatusBadRequest, "Invalid alertmanager choice specified", err)
 	}
 
+	disableExternal := srv.featureManager.IsEnabled(c.Req.Context(), featuremgmt.FlagAlertingDisableSendAlertsExternal)
+	if disableExternal && sendAlertsTo != ngmodels.InternalAlertmanager {
+		return response.Error(http.StatusBadRequest, "Sending alerts to external alertmanagers is disallowed on this instance", err)
+	}
+
 	externalAlertmanagers, err := srv.externalAlertmanagers(c.Req.Context(), c.SignedInUser.GetOrgID())
 	if err != nil {
 		return response.Error(http.StatusInternalServerError, "Couldn't fetch the external Alertmanagers from datasources", err)