fix(gateway client): Fix nil pointer dereference panic when using rul…

…er in ring mode (#13436)

Fixes gateway client return with nil pointer panic when using ring mode for ruler service.

`clients.dnsProvider` will only be init when using `simple mode` here: https://github.com/grafana/loki/blob/183406570411a5ad5ceaf32bf07451b8fce608c1/pkg/indexgateway/client.go#L184

Service like `ruler` using `ring mode`  will run into panic `nil pointer reference` during restarting or exiting:

pkg/indexgateway/client.go

@@ -210,7 +210,9 @@ func (s *GatewayClient) Stop() {
 	if err != nil {
 		level.Error(s.logger).Log("msg", "failed to stop index gateway connection pool", "err", err)
 	}
-	s.dnsProvider.Stop()
+	if s.cfg.Mode == SimpleMode {
+		s.dnsProvider.Stop()
+	}
 }
 
 func (s *GatewayClient) QueryPages(ctx context.Context, queries []index.Query, callback index.QueryPagesCallback) error {