@Override
    public List<Runnable> shutdownNow() {
        throw new UnsupportedOperationException();
    }
Keeps the {@link Future}s which were scheduled via this {@link ScheduledExecutorService}.
Note that this field is also used as a lock.
core/src/main/java/com/linecorp/armeria/client/endpoint/healthcheck/DefaultHealthCheckerContext.java