[grpclb] Use a separate channel arg to enable load reporting filter (#33465)

This avoids overloading the GRPC_ARG_LB_POLICY_NAME channel arg, which
is really intended for passing from the application to the client
channel.

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

@@ -153,6 +153,10 @@
 #define GRPC_GRPCLB_DEFAULT_FALLBACK_TIMEOUT_MS 10000
 #define GRPC_GRPCLB_DEFAULT_SUBCHANNEL_DELETION_DELAY_MS 10000
 
+// Channel arg used to enable load reporting filter.
+#define GRPC_ARG_GRPCLB_ENABLE_LOAD_REPORTING_FILTER \
+  "grpc.internal.grpclb_enable_load_reporting_filter"
+
 namespace grpc_core {
 
 TraceFlag grpc_lb_glb_trace(false, "glb");
@@ -503,7 +507,7 @@ class GrpcLb : public LoadBalancingPolicy {
   void ShutdownLocked() override;
 
   // Helper functions used in UpdateLocked().
-  absl::Status UpdateBalancerChannelLocked(const ChannelArgs& args);
+  absl::Status UpdateBalancerChannelLocked();
 
   void CancelBalancerChannelConnectivityWatchLocked();
 
@@ -1524,6 +1528,7 @@ absl::Status GrpcLb::UpdateLocked(UpdateArgs args) {
   const bool is_initial_update = lb_channel_ == nullptr;
   config_ = args.config;
   GPR_ASSERT(config_ != nullptr);
+  args_ = std::move(args.args);
   // Update fallback address list.
   fallback_backend_addresses_ = std::move(args.addresses);
   if (fallback_backend_addresses_.ok()) {
@@ -1536,7 +1541,7 @@ absl::Status GrpcLb::UpdateLocked(UpdateArgs args) {
   }
   resolution_note_ = std::move(args.resolution_note);
   // Update balancer channel.
-  absl::Status status = UpdateBalancerChannelLocked(args.args);
+  absl::Status status = UpdateBalancerChannelLocked();
   // Update the existing child policy, if any.
   if (child_policy_ != nullptr) CreateOrUpdateChildPolicyLocked();
   // If this is the initial update, start the fallback-at-startup checks
@@ -1577,12 +1582,9 @@ absl::Status GrpcLb::UpdateLocked(UpdateArgs args) {
 // helpers for UpdateLocked()
 //
 
-absl::Status GrpcLb::UpdateBalancerChannelLocked(const ChannelArgs& args) {
-  // Make sure that GRPC_ARG_LB_POLICY_NAME is set in channel args,
-  // since we use this to trigger the client_load_reporting filter.
-  args_ = args.Set(GRPC_ARG_LB_POLICY_NAME, "grpclb");
+absl::Status GrpcLb::UpdateBalancerChannelLocked() {
   // Get balancer addresses.
-  ServerAddressList balancer_addresses = ExtractBalancerAddresses(args);
+  ServerAddressList balancer_addresses = ExtractBalancerAddresses(args_);
   absl::Status status;
   if (balancer_addresses.empty()) {
     status = absl::UnavailableError("balancer address list must be non-empty");
@@ -1591,7 +1593,7 @@ absl::Status GrpcLb::UpdateBalancerChannelLocked(const ChannelArgs& args) {
   auto channel_credentials = channel_control_helper()->GetChannelCredentials();
   // Construct args for balancer channel.
   ChannelArgs lb_channel_args =
-      BuildBalancerChannelArgs(response_generator_.get(), args);
+      BuildBalancerChannelArgs(response_generator_.get(), args_);
   // Create balancer channel if needed.
   if (lb_channel_ == nullptr) {
     std::string uri_str = absl::StrCat("fake:///", server_name_);
@@ -1603,7 +1605,7 @@ absl::Status GrpcLb::UpdateBalancerChannelLocked(const ChannelArgs& args) {
     channelz::ChannelNode* child_channelz_node =
         grpc_channel_get_channelz_node(lb_channel_);
     channelz::ChannelNode* parent_channelz_node =
-        args.GetObject<channelz::ChannelNode>();
+        args_.GetObject<channelz::ChannelNode>();
     if (child_channelz_node != nullptr && parent_channelz_node != nullptr) {
       parent_channelz_node->AddChildChannel(child_channelz_node->uuid());
       parent_channelz_node_ = parent_channelz_node->Ref();
@@ -1728,8 +1730,10 @@ void GrpcLb::OnFallbackTimerLocked() {
 ChannelArgs GrpcLb::CreateChildPolicyArgsLocked(
     bool is_backend_from_grpclb_load_balancer) {
   ChannelArgs r =
-      args_.Set(GRPC_ARG_ADDRESS_IS_BACKEND_FROM_GRPCLB_LOAD_BALANCER,
-                is_backend_from_grpclb_load_balancer);
+      args_
+          .Set(GRPC_ARG_ADDRESS_IS_BACKEND_FROM_GRPCLB_LOAD_BALANCER,
+               is_backend_from_grpclb_load_balancer)
+          .Set(GRPC_ARG_GRPCLB_ENABLE_LOAD_REPORTING_FILTER, 1);
   if (is_backend_from_grpclb_load_balancer) {
     r = r.Set(GRPC_ARG_INHIBIT_HEALTH_CHECKING, 1);
   }
@@ -1881,8 +1885,9 @@ void RegisterGrpcLbPolicy(CoreConfiguration::Builder* builder) {
   builder->channel_init()->RegisterStage(
       GRPC_CLIENT_SUBCHANNEL, GRPC_CHANNEL_INIT_BUILTIN_PRIORITY,
       [](ChannelStackBuilder* builder) {
-        if (builder->channel_args().GetString(GRPC_ARG_LB_POLICY_NAME) ==
-            "grpclb") {
+        auto enable = builder->channel_args().GetBool(
+            GRPC_ARG_GRPCLB_ENABLE_LOAD_REPORTING_FILTER);
+        if (enable.has_value() && *enable) {
           builder->PrependFilter(&ClientLoadReportingFilter::kFilter);
         }
         return true;