private void startHealthChecker(Consumer<? super HealthChecker> listener) {
            if (state != State.INIT) {
                return;
            }
            state = State.SCHEDULED;
            settableHealthChecker.addListener(listener);
            scheduledFuture = eventExecutor.submit(this::runHealthCheck);
        }
Health checker can be scheduled only once. Calling startHealthChecker won't work after stopHealthChecker.
This class itself is not thread-safe and guarded by {@link ScheduledHealthChecker} to avoid calling
{@link ScheduledHealthChecker#startHealthChecker()} and
{@link ScheduledHealthChecker#stopHealthChecker()} concurrently.
core/src/main/java/com/linecorp/armeria/server/healthcheck/ScheduledHealthChecker.java