- [ ] 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
* <p>
     * see Filter
     */
    String[] filter() default {};
    /**
     * Listeners for service exporting and unexporting
     * <p>
     * see ExporterListener
     */
    String[] listener() default {};
    /**
     * Customized parameter key-value pair, for example: {key1, value1, key2, value2}
     */
    String[] parameters() default {};
    /**
     * Application associated name
     */
    String application() default "";
    /**
     * Module associated name
     */
    String module() default "";
    /**
     * Consumer associated name
     */
    String consumer() default "";
    /**
     * Monitor associated name
     */
    String monitor() default "";