- [ ] 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
*
 * @since 2.7.7
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.FIELD, ElementType.METHOD, ElementType.ANNOTATION_TYPE})
public @interface DubboReference {
    /**
     * Interface class, default value is void.class
     */
    Class<?> interfaceClass() default void.class;
    /**
     * Interface class name, default value is empty string
     */
    String interfaceName() default "";
    /**
     * Service version, default value is empty string
     */
    String version() default "";
    /**
     * Service group, default value is empty string
     */
    String group() default "";
    /**
     * Service target URL for direct invocation, if this is specified, then registry center takes no effect.
     */
    String url() default "";
    /**
     * Client transport type, default value is "netty"
     */