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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)



/**
                 * Get all instances with serviceNames to avoid instance overwrite and but with empty instance mentioned
                 * in https://github.com/apache/dubbo/issues/5885 and https://github.com/apache/dubbo/issues/5899
                 *
                 * namingService.getAllInstances with {@link org.apache.dubbo.registry.support.AbstractRegistry#registryUrl}
                 * default {@link DEFAULT_GROUP}
                 *
                 * in https://github.com/apache/dubbo/issues/5978
                 */
                for (String serviceName : serviceNames) {
                    List<Instance> instances = namingService.getAllInstances(serviceName,
                        getUrl().getGroup(Constants.DEFAULT_GROUP));
                    NacosInstanceManageUtil.initOrRefreshServiceInstanceList(serviceName, instances);
                    allCorrespondingInstanceList.addAll(instances);
                }
                notifySubscriber(url, listener, allCorrespondingInstanceList);
                for (String serviceName : serviceNames) {
                    subscribeEventListener(serviceName, url, listener);
                }
            } else {
                for (String serviceName : serviceNames) {
                    List<Instance> instances = new LinkedList<>();
                    instances.addAll(namingService.getAllInstances(serviceName
                        , getUrl().getGroup(Constants.DEFAULT_GROUP)));
                    String serviceInterface = serviceName;
                    String[] segments = serviceName.split(SERVICE_NAME_SEPARATOR, -1);
                    if (segments.length == 4) {
                        serviceInterface = segments[SERVICE_INTERFACE_INDEX];
                    }
                    URL subscriberURL = url.setPath(serviceInterface).addParameters(INTERFACE_KEY, serviceInterface,