<!-- If you need to report a security issue please visit https://github.com/apache/dubbo/security/policy -->

- [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)



import org.apache.dubbo.common.URLBuilder;
import org.apache.dubbo.common.logger.Logger;
import org.apache.dubbo.common.logger.LoggerFactory;
import org.apache.dubbo.common.url.component.DubboServiceAddressURL;
import org.apache.dubbo.common.url.component.ServiceConfigURL;
import org.apache.dubbo.common.utils.StringUtils;
import org.apache.dubbo.common.utils.UrlUtils;
import org.apache.dubbo.registry.NotifyListener;
import org.apache.dubbo.registry.Registry;
import org.apache.dubbo.registry.RegistryNotifier;
import org.apache.dubbo.registry.nacos.util.NacosInstanceManageUtil;
import org.apache.dubbo.registry.support.FailbackRegistry;
import org.apache.dubbo.rpc.RpcException;
import com.alibaba.nacos.api.common.Constants;
import com.alibaba.nacos.api.exception.NacosException;
import com.alibaba.nacos.api.naming.listener.Event;
import com.alibaba.nacos.api.naming.listener.EventListener;
import com.alibaba.nacos.api.naming.listener.NamingEvent;
import com.alibaba.nacos.api.naming.pojo.Instance;
import com.alibaba.nacos.api.naming.pojo.ListView;
import com.google.common.collect.Lists;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.HashMap;
import java.util.LinkedHashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Set;