- [ ] 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
/**
     * Whether to enable async invocation, default value is false
     */
    boolean async() default false;
    /**
     * Maximum active requests allowed, default value is 0
     */
    int actives() default 0;
    /**
     * Whether the async request has already been sent, the default value is false
     */
    boolean sent() default false;
    /**
     * Service mock name, use interface name + Mock if not set
     */
    String mock() default "";
    /**
     * Whether to use JSR303 validation, legal values are: true, false
     */
    String validation() default "";
    /**
     * Timeout value for service invocation, default value is 0
     */
    int timeout() default 0;
    /**
     * Specify cache implementation for service invocation, legal values include: lru, threadlocal, jcache
     */
    String cache() default "";
    /**
     * Filters for service invocation