[grpclb] set resolution note for empty list from balancer (#34487)

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src/core/ext/filters/client_channel/lb_policy/grpclb/grpclb.cc

@@ -1767,6 +1767,9 @@ void GrpcLb::CreateOrUpdateChildPolicyLocked() {
     update_args.addresses = serverlist_->GetServerAddressList(
         lb_calld_ == nullptr ? nullptr : lb_calld_->client_stats());
     is_backend_from_grpclb_load_balancer = true;
+    if (update_args.addresses.ok() && update_args.addresses->empty()) {
+      update_args.resolution_note = "empty serverlist from grpclb balancer";
+    }
   }
   update_args.args =
       CreateChildPolicyArgsLocked(is_backend_from_grpclb_load_balancer);