- [ ] 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
import java.lang.annotation.Target;
/**
 * @since 2.6.5
 *  *
 *  * 2018/9/29
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.ANNOTATION_TYPE})
@Inherited
public @interface Method {
    String name();
    int timeout() default -1;
    int retries() default -1;
    String loadbalance() default "";
    boolean async() default false;
    boolean sent() default true;
    int actives() default 0;
    int executes() default 0;
    boolean deprecated() default false;
    boolean sticky() default false;
    boolean isReturn() default true;
    String oninvoke() default "";
    String onreturn() default "";
    String onthrow() default "";
    String cache() default "";
    String validation() default "";
    Argument[] arguments() default {};
}