GuardedScheduler(Scheduler delegate) {
    this.delegate = requireNonNull(delegate);
  }
@Test
  public void guardedScheduler() {
    var future = Scheduler.guardedScheduler((r, e, d, u) -> Futures.immediateVoidFuture())
        .schedule(Runnable::run, () -> {}, 1, TimeUnit.MINUTES);
    assertThat(future).isSameInstanceAs(Futures.immediateVoidFuture());
  }