[tracing] Add target_write_size to HttpAnnotation (#35921)

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Closes #35921

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35921 from yousukseung:wire-size-target 6c01052721
PiperOrigin-RevId: 607822267

src/core/ext/transport/chttp2/transport/chttp2_transport.cc

@@ -338,16 +338,11 @@ void ForEachContextListEntryExecute(void* arg, Timestamps* ts,
   delete context_list;
 }
 
-HttpAnnotation::HttpAnnotation(
-    Type type, gpr_timespec time,
-    absl::optional<chttp2::TransportFlowControl::Stats> transport_stats,
-    absl::optional<chttp2::StreamFlowControl::Stats> stream_stats)
+HttpAnnotation::HttpAnnotation(Type type, gpr_timespec time)
     : CallTracerAnnotationInterface::Annotation(
           CallTracerAnnotationInterface::AnnotationType::kHttpTransport),
       type_(type),
-      time_(time),
-      transport_stats_(transport_stats),
-      stream_stats_(stream_stats) {}
+      time_(time) {}
 
 std::string HttpAnnotation::ToString() const {
   std::string s = "HttpAnnotation type: ";
@@ -1433,9 +1428,11 @@ static void perform_stream_op_locked(void* stream_op,
 
   if (op->send_initial_metadata) {
     if (s->call_tracer) {
-      s->call_tracer->RecordAnnotation(grpc_core::HttpAnnotation(
-          grpc_core::HttpAnnotation::Type::kStart, gpr_now(GPR_CLOCK_REALTIME),
-          s->t->flow_control.stats(), s->flow_control.stats()));
+      s->call_tracer->RecordAnnotation(
+          grpc_core::HttpAnnotation(grpc_core::HttpAnnotation::Type::kStart,
+                                    gpr_now(GPR_CLOCK_REALTIME))
+              .Add(s->t->flow_control.stats())
+              .Add(s->flow_control.stats()));
     }
     if (t->is_client && t->channelz_socket != nullptr) {
       t->channelz_socket->RecordStreamStartedFromLocal();

src/core/ext/transport/chttp2/transport/chttp2_transport.h

@@ -112,10 +112,27 @@ class HttpAnnotation : public CallTracerAnnotationInterface::Annotation {
     kEnd,
   };
 
-  HttpAnnotation(
-      Type type, gpr_timespec time,
-      absl::optional<chttp2::TransportFlowControl::Stats> transport_stats,
-      absl::optional<chttp2::StreamFlowControl::Stats> stream_stats);
+  // A snapshot of write stats to export.
+  struct WriteStats {
+    size_t target_write_size;
+  };
+
+  HttpAnnotation(Type type, gpr_timespec time);
+
+  HttpAnnotation& Add(const chttp2::TransportFlowControl::Stats& stats) {
+    transport_stats_ = stats;
+    return *this;
+  }
+
+  HttpAnnotation& Add(const chttp2::StreamFlowControl::Stats& stats) {
+    stream_stats_ = stats;
+    return *this;
+  }
+
+  HttpAnnotation& Add(const WriteStats& stats) {
+    write_stats_ = stats;
+    return *this;
+  }
 
   std::string ToString() const override;
 
@@ -133,6 +150,7 @@ class HttpAnnotation : public CallTracerAnnotationInterface::Annotation {
   const gpr_timespec time_;
   absl::optional<chttp2::TransportFlowControl::Stats> transport_stats_;
   absl::optional<chttp2::StreamFlowControl::Stats> stream_stats_;
+  absl::optional<WriteStats> write_stats_;
 };
 
 }  // namespace grpc_core

src/core/ext/transport/chttp2/transport/writing.cc

@@ -492,10 +492,15 @@ class StreamWriteContext {
         t_, s_, &s_->send_initial_metadata_finished, absl::OkStatus(),
         "send_initial_metadata_finished");
     if (s_->call_tracer) {
-      s_->call_tracer->RecordAnnotation(grpc_core::HttpAnnotation(
-          grpc_core::HttpAnnotation::Type::kHeadWritten,
-          gpr_now(GPR_CLOCK_REALTIME), s_->t->flow_control.stats(),
-          s_->flow_control.stats()));
+      grpc_core::HttpAnnotation::WriteStats write_stats;
+      write_stats.target_write_size = write_context_->target_write_size();
+      s_->call_tracer->RecordAnnotation(
+          grpc_core::HttpAnnotation(
+              grpc_core::HttpAnnotation::Type::kHeadWritten,
+              gpr_now(GPR_CLOCK_REALTIME))
+              .Add(s_->t->flow_control.stats())
+              .Add(s_->flow_control.stats())
+              .Add(write_stats));
     }
   }
 
@@ -619,9 +624,11 @@ class StreamWriteContext {
     grpc_chttp2_mark_stream_closed(t_, s_, !t_->is_client, true,
                                    absl::OkStatus());
     if (s_->call_tracer) {
-      s_->call_tracer->RecordAnnotation(grpc_core::HttpAnnotation(
-          grpc_core::HttpAnnotation::Type::kEnd, gpr_now(GPR_CLOCK_REALTIME),
-          s_->t->flow_control.stats(), s_->flow_control.stats()));
+      s_->call_tracer->RecordAnnotation(
+          grpc_core::HttpAnnotation(grpc_core::HttpAnnotation::Type::kEnd,
+                                    gpr_now(GPR_CLOCK_REALTIME))
+              .Add(s_->t->flow_control.stats())
+              .Add(s_->flow_control.stats()));
     }
   }