private void rejectIfDestroyed(Object command) {
        if (destroyed) {
            throw new RejectedExecutionException(
                    HealthCheckerContext.class.getSimpleName() + " for '" + endpoint +
                    "' has been destroyed already. Task: " + command);
        }
    }
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