- [ ] 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
String client() default "";
    /**
     * Whether to enable generic invocation, default value is false
     */
    boolean generic() default false;
    /**
     * When enable, prefer to call local service in the same JVM if it's present, default value is true
     */
    boolean injvm() default true;
    /**
     * Check if service provider is available during boot up, default value is true
     */
    boolean check() default true;
    /**
     * Whether eager initialize the reference bean when all properties are set, default value is false
     */
    boolean init() default false;
    /**
     * Whether to make connection when the client is created, the default value is false
     */
    boolean lazy() default false;
    /**
     * Export an stub service for event dispatch, default value is false.
     * <p>
     * see org.apache.dubbo.rpc.Constants#STUB_EVENT_METHODS_KEY
     */
    boolean stubevent() default false;
    /**
     * Whether to reconnect if connection is lost, if not specify, reconnect is enabled by default, and the interval
     * for retry connecting is 2000 ms