SampleImpl target = new SampleImpl(DELAY_MS);
    Sample proxy = service.newProxy(target, Sample.class, ENOUGH_MS, MILLISECONDS);
    Stopwatch stopwatch = Stopwatch.createStarted();

    String result = proxy.sleepThenReturnInput("x");

    assertThat(result).isEqualTo("x");
    assertThat(stopwatch.elapsed(MILLISECONDS)).isIn(Range.closed(DELAY_MS, ENOUGH_MS));
    assertThat(target.finished).isTrue();
  }