message: Do not record queue length in metrics on task dequeue.
  It is more meaningful for the metrics to contain the queue length on enqueue tasks only.
diff: @@ -440,8 +440,7 @@ void ExecutorState<PropagatorStateType>::RunTask(Closure&& c) {
  // mutable is needed because std::forward<Closure> in the lambda body may move
  // the Closure `c`.
  runner_([c = std::forward<Closure>(c)]() mutable {
  - auto qlen = queue_length.fetch_sub(1, std::memory_order_relaxed);
  - metrics::UpdateGraphPendingQueueLength(qlen - 1);
  + queue_length.fetch_sub(1, std::memory_order_relaxed);
  std::forward<Closure>(c)();
  });
  }
  