<T extends XdsResource> boolean register(XdsType type, String resourceName, ResourceWatcher<T> watcher) {
        //noinspection unchecked
        XdsStreamSubscriber<T> subscriber = (XdsStreamSubscriber<T>) subscriberMap.computeIfAbsent(
                type, key -> new HashMap<>()).get(resourceName);
        boolean updated = false;
        if (subscriber == null) {
            subscriber = new XdsStreamSubscriber<>(type, resourceName, eventLoop, timeoutMillis);
            subscriberMap.get(type).put(resourceName, subscriber);
            updated = true;
        }
        subscriber.registerWatcher(watcher);
        return updated;
    }
Returns {@code true} if a new subscriber is added.
xds/src/main/java/com/linecorp/armeria/xds/SubscriberStorage.java