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- [x] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 2.7.14、3.0.4

### Steps to reproduce this issue

1. start dubbo admin (nacos registry)

The ```EventListener``` instance has created countless times by the scheduled task(```scheduledExecutorService```) and cached in the ```InstancesChangeNotifier#listenerMap```(Nacos)  , until the memory is full.

- NacosRegistry
```
private void subscribeEventListener(String serviceName, final URL url, final NotifyListener listener)
        throws NacosException {
        EventListener eventListener = new RegistryChildListenerImpl(serviceName, url, listener);
        namingService.subscribe(serviceName,
            getUrl().getGroup(Constants.DEFAULT_GROUP),
            eventListener);
}
```

- InstancesChangeNotifier
```
private final Map<String, ConcurrentHashSet<EventListener>> listenerMap = new ConcurrentHashMap<String, ConcurrentHashSet<EventListener>>();

public void registerListener(String serviceName, String clusters, EventListener listener) {
        String key = ServiceInfo.getKey(serviceName, clusters);
        ConcurrentHashSet<EventListener> eventListeners = listenerMap.get(key);
        if (eventListeners == null) {
            synchronized (lock) {
                eventListeners = listenerMap.get(key);
                if (eventListeners == null) {
                    eventListeners = new ConcurrentHashSet<EventListener>();
                    listenerMap.put(key, eventListeners);
                }
            }
        }
        eventListeners.add(listener);
}
```
The ```namingService.subscribe```  is finally called to the ```InstancesChangeNotifier#registerListener``` , so 
eventListener cached in  eventListeners.

![image](https://user-images.githubusercontent.com/5037807/140080041-f617bda2-0f48-4669-8e3d-2184a6eb9a75.png)



throw new IllegalArgumentException("lookup url == null");
        }
        try {
            List<URL> urls = new LinkedList<>();
            Set<String> serviceNames = getServiceNames(url, null);
            for (String serviceName : serviceNames) {
                List<Instance> instances = namingService.getAllInstances(serviceName,
                    getUrl().getGroup(Constants.DEFAULT_GROUP));
                urls.addAll(buildURLs(url, instances));
            }
            return urls;
        } catch (Throwable cause) {
            throw new RpcException("Failed to lookup " + url + " from nacos " + getUrl() + ", cause: " + cause.getMessage(), cause);
        }
    }
    @Override
    public void doRegister(URL url) {
        try {
            String serviceName = getServiceName(url);
            Instance instance = createInstance(url);
            /**
             *  namingService.registerInstance with {@link org.apache.dubbo.registry.support.AbstractRegistry#registryUrl}
             *  default {@link DEFAULT_GROUP}
             *
             * in https://github.com/apache/dubbo/issues/5978
             */
            namingService.registerInstance(serviceName,
                getUrl().getGroup(Constants.DEFAULT_GROUP), instance);
        } catch (Throwable cause) {
            throw new RpcException("Failed to register " + url + " to nacos " + getUrl() + ", cause: " + cause.getMessage(), cause);