delete tenant from skipped list if it's not owned by the instance, al…

…ways use the default config in startAlertmanager()

pkg/alertmanager/multitenant.go

@@ -1124,19 +1124,12 @@ func (am *MultitenantAlertmanager) serveRequest(w http.ResponseWriter, req *http
 func (am *MultitenantAlertmanager) startAlertmanager(ctx context.Context, userID string) (*Alertmanager, error) {
 	// Avoid starting the Alertmanager for tenants not owned by this instance.
 	if !am.isUserOwned(userID) {
+		am.lastRequestTime.Delete(userID)
 		return nil, errors.Wrap(errNotUploadingFallback, "user not owned by this instance")
 	}
 
-	cfg, err := am.store.GetAlertConfig(ctx, userID)
-	if err != nil {
-		if !errors.Is(err, alertspb.ErrNotFound) {
-			return nil, errors.Wrap(err, "failed to check for existing configuration")
-		}
-		cfg = alertspb.ToProto("", nil, userID)
-	}
-
 	amConfig := amConfig{
-		AlertConfigDesc: cfg,
+		AlertConfigDesc: alertspb.ToProto("", nil, userID),
 		tmplExternalURL: am.cfg.ExternalURL.URL,
 	}
 	if err := am.setConfig(amConfig); err != nil {