- [ ] 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
* <p>
     * see org.apache.dubbo.remoting.Constants#DEFAULT_RECONNECT_PERIOD
     */
    String reconnect() default "";
    /**
     * Whether to stick to the same node in the cluster, the default value is false
     * <p>
     * see Constants#DEFAULT_CLUSTER_STICKY
     */
    boolean sticky() default false;
    /**
     * How the proxy is generated, legal values include: jdk, javassist
     */
    String proxy() default "";
    /**
     * Service stub name, use interface name + Local if not set
     */
    String stub() default "";
    /**
     * Cluster strategy, legal values include: failover, failfast, failsafe, failback, forking
     */
    String cluster() default "";
    /**
     * Maximum connections service provider can accept, default value is 0 - connection is shared
     */
    int connections() default 0;
    /**
     * The callback instance limit peer connection
     * <p>
     * see org.apache.dubbo.rpc.Constants#DEFAULT_CALLBACK_INSTANCES