[WRR] reduce picker memory usage (#33086)

Instead of copying the individual fields from the LB policy config into
the picker, we just take a ref to the config.

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

@@ -340,11 +340,7 @@ class WeightedRoundRobin : public LoadBalancingPolicy {
         ABSL_EXCLUSIVE_LOCKS_REQUIRED(&timer_mu_);
 
     RefCountedPtr<WeightedRoundRobin> wrr_;
-    const bool use_per_rpc_utilization_;
-    const Duration weight_update_period_;
-    const Duration weight_expiration_period_;
-    const Duration blackout_period_;
-    const float error_utilization_penalty_;
+    RefCountedPtr<WeightedRoundRobinConfig> config_;
     std::vector<SubchannelInfo> subchannels_;
 
     Mutex scheduler_mu_;
@@ -505,11 +501,7 @@ WeightedRoundRobin::Picker::Picker(
     RefCountedPtr<WeightedRoundRobin> wrr,
     WeightedRoundRobinSubchannelList* subchannel_list)
     : wrr_(std::move(wrr)),
-      use_per_rpc_utilization_(!wrr_->config_->enable_oob_load_report()),
-      weight_update_period_(wrr_->config_->weight_update_period()),
-      weight_expiration_period_(wrr_->config_->weight_expiration_period()),
-      blackout_period_(wrr_->config_->blackout_period()),
-      error_utilization_penalty_(wrr_->config_->error_utilization_penalty()),
+      config_(wrr_->config_),
       last_picked_index_(absl::Uniform<size_t>(wrr_->bit_gen_)) {
   for (size_t i = 0; i < subchannel_list->num_subchannels(); ++i) {
     WeightedRoundRobinSubchannelData* sd = subchannel_list->subchannel(i);
@@ -548,9 +540,9 @@ WeightedRoundRobin::PickResult WeightedRoundRobin::Picker::Pick(
   auto& subchannel_info = subchannels_[index];
   // Collect per-call utilization data if needed.
   std::unique_ptr<SubchannelCallTrackerInterface> subchannel_call_tracker;
-  if (use_per_rpc_utilization_) {
+  if (!config_->enable_oob_load_report()) {
     subchannel_call_tracker = std::make_unique<SubchannelCallTracker>(
-        subchannel_info.weight, error_utilization_penalty_);
+        subchannel_info.weight, config_->error_utilization_penalty());
   }
   if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_wrr_trace)) {
     gpr_log(GPR_INFO,
@@ -582,7 +574,7 @@ void WeightedRoundRobin::Picker::BuildSchedulerAndStartTimerLocked() {
   weights.reserve(subchannels_.size());
   for (const auto& subchannel : subchannels_) {
     weights.push_back(subchannel.weight->GetWeight(
-        now, weight_expiration_period_, blackout_period_));
+        now, config_->weight_expiration_period(), config_->blackout_period()));
   }
   if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_wrr_trace)) {
     gpr_log(GPR_INFO, "[WRR %p picker %p] new weights: %s", wrr_.get(), this,
@@ -609,7 +601,7 @@ void WeightedRoundRobin::Picker::BuildSchedulerAndStartTimerLocked() {
   // Start timer.
   WeakRefCountedPtr<Picker> self = WeakRef();
   timer_handle_ = wrr_->channel_control_helper()->GetEventEngine()->RunAfter(
-      weight_update_period_, [self = std::move(self)]() mutable {
+      config_->weight_update_period(), [self = std::move(self)]() mutable {
         ApplicationCallbackExecCtx callback_exec_ctx;
         ExecCtx exec_ctx;
         {