[chttp2] Fix a bug whereby trailers-only is not reported (#31537)

We don't report trailers-only if the trailers are parsed prior to executing recv_initial_metadata

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

@@ -1395,6 +1395,9 @@ static void perform_stream_op_locked(void* stream_op,
         op_payload->recv_initial_metadata.recv_initial_metadata;
     s->trailing_metadata_available =
         op_payload->recv_initial_metadata.trailing_metadata_available;
+    if (s->parsed_trailers_only && s->trailing_metadata_available != nullptr) {
+      *s->trailing_metadata_available = true;
+    }
     if (op_payload->recv_initial_metadata.peer_string != nullptr) {
       gpr_atm_rel_store(op_payload->recv_initial_metadata.peer_string,
                         (gpr_atm)t->peer_string.c_str());

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

@@ -514,6 +514,7 @@ struct grpc_chttp2_stream {
   grpc_metadata_batch* recv_initial_metadata;
   grpc_closure* recv_initial_metadata_ready = nullptr;
   bool* trailing_metadata_available = nullptr;
+  bool parsed_trailers_only = false;
   absl::optional<grpc_core::SliceBuffer>* recv_message = nullptr;
   uint32_t* recv_message_flags = nullptr;
   bool* call_failed_before_recv_message = nullptr;

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

@@ -595,6 +595,7 @@ static grpc_error_handle init_header_frame_parser(grpc_chttp2_transport* t,
         if (s->trailing_metadata_available != nullptr) {
           *s->trailing_metadata_available = true;
         }
+        s->parsed_trailers_only = true;
         incoming_metadata_buffer = &s->trailing_metadata_buffer;
         frame_type = HPackParser::LogInfo::kTrailers;
       } else {