2.x能获取实例注册时间，3.x功能丢失

### Problem this feature would solve

org.apache.servicecomb.registry.api.MicroserviceInstance接口原本能获取实例注册时间，我们可根据注册时间进行预热操作，升级3.x之后只能反射获取了，不太友好，麻烦开放一下获取实例timestamp参数

### Describe the solution

_No response_

### Alternatives considered

_No response_

### Additional Context

_No response_

Start with the org.apache.servicecomb.registry.api.MicroserviceInstance interface and compare its 2.x and 3.x APIs to determine how the instance registration timestamp was previously exposed. Confirm the public getter and its expected value, then verify that callers can obtain the timestamp without reflection.

Written by the indexing model from the issue text.