public void addConfigListener(AbstractInterfaceConfig config, ConfigListener listener) {
        if (listener != null) {
            RegistryUtils.initOrAddList(configListenerMap, config, listener);
        }
    }
Add config listener.

@param config   the config
@param listener the listener
registry/registry-zk/src/main/java/com/alipay/sofa/rpc/registry/zk/ZookeeperOverrideObserver.java