message: [tf.data service] Add additional traceme information.
diff: @@ -304,6 +304,26 @@ class DataServiceDatasetOp::Dataset : public DatasetBase {
  return errors::Unimplemented("RestoreInternal is not yet supported");
  }
  + data::TraceMeMetadata GetTraceMeMetadata() const override {
  + data::TraceMeMetadata result;
  + int64 num_tasks = -1;
  + if (mu_.try_lock()) {
  + num_tasks = tasks_.size() - finished_tasks_;
  + mu_.unlock();
  + }
  + std::string num_tasks_string =
  + (num_tasks == -1)
  + ? "unavailable"
  + : strings::Printf("%lld", static_cast<long long>(num_tasks));
  + result.push_back(std::make_pair("num_tasks", num_tasks_string));
  + result.push_back(std::make_pair("job_name", dataset()->job_name_));
  + result.push_back(std::make_pair(
  + "max_outstanding_requests",
  + strings::Printf("%lld", static_cast<long long>(
  + dataset()->max_outstanding_requests_))));
  + return result;
  + }
  +
  private:
  struct Task {
  Task(int64 task_id, const std::string& address,
  @@ -571,7 +591,7 @@ class DataServiceDatasetOp::Dataset : public DatasetBase {
  const int64 iterator_index_;
  - mutex mu_;
  + mutable mutex mu_;
  condition_variable get_next_cv_ TF_GUARDED_BY(mu_);
  condition_variable worker_thread_cv_ TF_GUARDED_BY(mu_);
  condition_variable manager_thread_cv_ TF_GUARDED_BY(mu_);
  