CompletableFuture API err: System Error, please try again later! Message:dubbo 2.6 does not support metadata

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

### Environment

* Dubbo Admin version: recent
* Operating System version: mac
* Java version: 1.8

### Steps to reproduce this issue

define api:
```java

public interface HelloService {

CompletableFuture sayHelloAsync(String name);
}

```

then run server:

`System Error, please try again later! Message:dubbo 2.6 does not support metadata `

### Expected Result

What do you expected from the above steps？

### Actual Result

What actually happens?

If there is an exception, please attach the exception trace:

```
Just put your stack trace here!
```

Start by reproducing the CompletableFuture service example with Dubbo 2.6 and inspect the metadata-related error path. The issue provides no source file or test location; done should mean the reported compatibility failure is resolved or its supported-version behavior is clearly verified.

Written by the indexing model from the issue text.