private void stopHealthChecker(Consumer<?> listener) {
            if (state != State.SCHEDULED) {
                return;
            }
            state = State.FINISHED;
            scheduledFuture.cancel(true);
            settableHealthChecker.removeListener(listener);
        }
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