Method TryAcquire

cpp/src/arrow/util/async_util_test.cc:350–356

class CustomThrottle : public ThrottledAsyncTaskScheduler::Throttle {
 public:
  virtual std::optional<Future<>> TryAcquire(int amt) {
    if (gate_.is_finished()) {
      return std::nullopt;
    } else {
      return gate_;
    }
  }
  virtual void Release(int amt) {}
  void Unlock() { gate_.MarkFinished(); }
  int Capacity() { return std::numeric_limits<int>::max(); }