Does MetadataService really need the external Activate filter chain? And there is no filter option to disable it.

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

### Environment

* Dubbo version: the 3.0 branch of source code
* Operating System version: Mac M1
* Java version: 1.8

### Optional Solution
#### 1. It's just fine
Do nothing.

#### 2. Disable external `@Activate` filter for `org.apache.dubbo.metadata.MetadataService`
In the solution, we consider `MetadataService` as Internal serivcie, so we forbid user intercepting it.
We can archive it in `ProtocolFilterWrapper#export` and `ProtocolFilterWrapper#export` methods like `isRegistry` did.

```java
// => org.apache.dubbo.rpc.cluster.filter.ProtocolFilterWrapper

@Override
public Exporter export(Invoker invoker) throws RpcException {
if (UrlUtils.isRegistry(invoker.getUrl())) {
return protocol.export(invoker);
}
return protocol.export(builder.buildInvokerChain(invoker, SERVICE_FILTER_KEY, CommonConstants.PROVIDER));
}

@Override
public Invoker refer(Class type, URL url) throws RpcException {
if (UrlUtils.isRegistry(url)) {
return protocol.refer(type, url);
}
return builder.buildInvokerChain(protocol.refer(type, url), REFERENCE_FILTER_KEY, CommonConstants.CONSUMER);
}
```

#### 3. Add filter option for MetadataService as provider and consumer do.
If MetadataService really needs the external Activate filter chain, but I'm doubtful about it.

Start with org.apache.dubbo.rpc.cluster.filter.ProtocolFilterWrapper#export and #refer, then trace how MetadataService is handled and how UrlUtils.isRegistry bypasses the filter chain. Review the three proposed behaviors and the existing provider/consumer filter options. Done means the chosen behavior is implemented and MetadataService filtering is verified for both export and reference paths.

Written by the indexing model from the issue text.