- [ ] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.
- [ ] 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: 2.7.5
* Operating System version: win10
* Java version: 1.8

### Steps to reproduce this issue

1. 基于xml的方式可以对分组方法进行merger调用
![image](https://user-images.githubusercontent.com/33019873/80855389-77f69d80-8c73-11ea-9b46-045fe8819092.png)
2. 但是基于注解方式`@Reference`或`@Method`都不支持指定merger

### Expected Result

可以通过`@Reference`或`@Method`指定merger
/**
     * Registry associated name
     */
    String[] registry() default {};
    /**
     * The communication protocol of Dubbo Service
     *
     * @return the default value is ""
     * @since 2.6.6
     */
    String protocol() default "";
    /**
     * Service tag name
     */
    String tag() default "";
    /**
     * methods support
     *
     * @return
     */
    Method[] methods() default {};
    /**
     * The id
     *
     * @return default value is empty
     * @since 2.7.3
     */
    String id() default "";
}