message: [Executor tracing] Add num_output_edges to the PropagateOutputs TraceMe.
  This enables someone reading a trace to distinguish between long PropagateOutputs events that are due to contention, and long events that are due to a lot of propagation work going on.
diff: @@ -2161,7 +2161,9 @@ void ExecutorState::PropagateOutputs(const TaggedNode& tagged_node,
  profiler::TraceMe activity(
  [&]() {
  return strings::StrCat("ExecutorPropagateOutputs#", "id=", step_id_,
  - ",kernel_name=", item->kernel->name_view(), "#");
  + ",kernel_name=", item->kernel->name_view(),
  + ",num_output_edges=", item->num_output_edges,
  + "#");
  },
  profiler::GetTFTraceMeLevel(/*is_expensive=*/false));
  