@Nullable
    CompletableFuture<?> release() {
        assert refCnt > 0 : refCnt;

        if (--refCnt == 0) {
            return destroy();
        }
        return null;
    }
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