[context] Move backend metric provider to arena based context (#36781)

Closes #36781

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36781 from ctiller:ctx7 06f44a31d1
PiperOrigin-RevId: 639185505

src/core/BUILD

@@ -5032,6 +5032,7 @@ grpc_cc_library(
         "ext/filters/backend_metrics/backend_metric_provider.h",
     ],
     language = "c++",
+    deps = ["arena"],
 )
 
 grpc_cc_library(

src/core/ext/filters/backend_metrics/backend_metric_filter.cc

@@ -130,16 +130,14 @@ BackendMetricFilter::Create(const ChannelArgs&, ChannelFilter::Args) {
 
 void BackendMetricFilter::Call::OnServerTrailingMetadata(ServerMetadata& md) {
   if (md.get(GrpcCallWasCancelled()).value_or(false)) return;
-  auto* ctx = &GetContext<
-      grpc_call_context_element>()[GRPC_CONTEXT_BACKEND_METRIC_PROVIDER];
+  auto* ctx = MaybeGetContext<BackendMetricProvider>();
   if (ctx == nullptr) {
     if (GRPC_TRACE_FLAG_ENABLED(grpc_backend_metric_filter_trace)) {
       gpr_log(GPR_INFO, "[%p] No BackendMetricProvider.", this);
     }
     return;
   }
-  absl::optional<std::string> serialized = MaybeSerializeBackendMetrics(
-      reinterpret_cast<BackendMetricProvider*>(ctx->value));
+  absl::optional<std::string> serialized = MaybeSerializeBackendMetrics(ctx);
   if (serialized.has_value() && !serialized->empty()) {
     if (GRPC_TRACE_FLAG_ENABLED(grpc_backend_metric_filter_trace)) {
       gpr_log(GPR_INFO, "[%p] Backend metrics serialized. size: %" PRIuPTR,

src/core/ext/filters/backend_metrics/backend_metric_provider.h

@@ -15,6 +15,8 @@
 #ifndef GRPC_SRC_CORE_EXT_FILTERS_BACKEND_METRICS_BACKEND_METRIC_PROVIDER_H
 #define GRPC_SRC_CORE_EXT_FILTERS_BACKEND_METRICS_BACKEND_METRIC_PROVIDER_H
 
+#include "src/core/lib/resource_quota/arena.h"
+
 namespace grpc_core {
 
 struct BackendMetricData;
@@ -24,6 +26,11 @@ class BackendMetricProvider {
   virtual BackendMetricData GetBackendMetricData() = 0;
 };
 
+template <>
+struct ArenaContextType<BackendMetricProvider> {
+  static void Destroy(BackendMetricProvider*) {}
+};
+
 }  // namespace grpc_core
 
 #endif  // GRPC_SRC_CORE_EXT_FILTERS_BACKEND_METRICS_BACKEND_METRIC_PROVIDER_H

src/core/lib/channel/context.h

@@ -32,10 +32,6 @@ typedef enum {
   /// Holds a pointer to ServiceConfigCallData associated with this call.
   GRPC_CONTEXT_SERVICE_CONFIG_CALL_DATA,
 
-  /// Holds a pointer to BackendMetricProvider associated with this call on
-  /// the server.
-  GRPC_CONTEXT_BACKEND_METRIC_PROVIDER,
-
   GRPC_CONTEXT_COUNT
 } grpc_context_index;
 

src/cpp/server/server_context.cc

@@ -410,8 +410,7 @@ void ServerContextBase::CreateCallMetricRecorder(
   auto* backend_metric_state =
       arena->New<BackendMetricState>(server_metric_recorder);
   call_metric_recorder_ = backend_metric_state;
-  grpc_call_context_set(call_.call, GRPC_CONTEXT_BACKEND_METRIC_PROVIDER,
-                        backend_metric_state, nullptr);
+  arena->SetContext<grpc_core::BackendMetricProvider>(backend_metric_state);
 }
 
 grpc::string_ref ServerContextBase::ExperimentalGetAuthority() const {