<!-- If you need to report a security issue please visit https://github.com/apache/dubbo/security/policy -->

- [x] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.

## Ask your question here
操作步骤如下：
1. k8s环境下，consumer端开启mesh模式
2. consumer端需要调用多个provider
3. provider有应用级注册和接口级注册，并且需要的provider不再同一个namespace下
4. 原有项目存在dubbo2.6与3.1多版本间的相互调用，在开启消费端mesh（dubbo3.1）后，无法调用到2.6的服务提供者

**问题：**

1. 如何实现在一个consumer下，调用多个namespace下的provider，并且provider有应用级注册和接口级注册
2. dubbo3.1消费端在开启mesh后，如何调用版本是2.6的服务提供者
String ROUTE_SCRIPT_PROTOCOL = "script";
    String OVERRIDE_PROTOCOL = "override";
    String COMPATIBLE_CONFIG_KEY = "compatible_config";
    String REGISTER_MODE_KEY = "register-mode";
    String DUBBO_REGISTER_MODE_DEFAULT_KEY = "dubbo.application.register-mode";
    String DUBBO_PUBLISH_INTERFACE_DEFAULT_KEY = "dubbo.application.publish-interface";
    String DUBBO_PUBLISH_INSTANCE_DEFAULT_KEY = "dubbo.application.publish-instance";
    String DEFAULT_REGISTER_MODE_INTERFACE = "interface";
    String DEFAULT_REGISTER_MODE_INSTANCE = "instance";
    String DEFAULT_REGISTER_MODE_ALL = "all";
    /**
     * The parameter key of Dubbo Registry type
     *
     * @since 2.7.5
     */
    String REGISTRY_TYPE_KEY = "registry-type";
    /**
     * The parameter value of Service-Oriented Registry type
     *
     * @since 2.7.5
     */
    String SERVICE_REGISTRY_TYPE = "service";
    /**
     * The protocol for Service Discovery
     *
     * @since 2.7.5
     */
    String SERVICE_REGISTRY_PROTOCOL = "service-discovery-registry";
    /**
     * Specify registry level services consumer needs to subscribe to, multiple values should be separated using ",".