- [ ] 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
*/
    int callbacks() default 0;
    /**
     * Callback method name when connected, default value is empty string
     */
    String onconnect() default "";
    /**
     * Callback method name when disconnected, default value is empty string
     */
    String ondisconnect() default "";
    /**
     * Service owner, default value is empty string
     */
    String owner() default "";
    /**
     * Service layer, default value is empty string
     */
    String layer() default "";
    /**
     * Service invocation retry times
     * <p>
     * see Constants#DEFAULT_RETRIES
     */
    int retries() default 2;
    /**
     * Load balance strategy, legal values include: random, roundrobin, leastactive
     * <p>
     * see Constants#DEFAULT_LOADBALANCE
     */
    String loadbalance() default "";