message: use reference value for annotation/kernel details xstat.
diff: @@ -85,7 +85,7 @@ void CreateXEvent(const CuptiTracerEvent& event, XPlaneBuilder* plane,
  if (!event.annotation.empty()) {
  xevent.AddStatValue(*plane->GetOrCreateStatMetadata(
  GetStatTypeStr(StatType::kKernelAnnotation)),
  - event.annotation);
  + *plane->GetOrCreateStatMetadata(event.annotation));
  }
  if (event.context_id != CuptiTracerEvent::kInvalidContextId) {
  xevent.AddStatValue(
  @@ -102,7 +102,7 @@ void CreateXEvent(const CuptiTracerEvent& event, XPlaneBuilder* plane,
  event.kernel_info.block_y, event.kernel_info.block_z);
  xevent.AddStatValue(*plane->GetOrCreateStatMetadata(
  GetStatTypeStr(StatType::kKernelDetails)),
  - kernel_details);
  + *plane->GetOrCreateStatMetadata(kernel_details));
  } else if (event.type == CuptiTracerEventType::MemcpyH2D ||
  event.type == CuptiTracerEventType::MemcpyD2H ||
  event.type == CuptiTracerEventType::MemcpyD2D ||
  @@ -145,7 +145,7 @@ void CreateXEvent(const CuptiTracerEvent& event, XPlaneBuilder* plane,
  if (!annotation_stack.empty()) {
  xevent.AddStatValue(
  *plane->GetOrCreateStatMetadata(GetStatTypeStr(StatType::kLevel0)),
  - annotation_stack.begin()->name);
  + *plane->GetOrCreateStatMetadata(annotation_stack.begin()->name));
  }
  }
  