see related task MDEV-15450

Why

In original, one MariaDB server instance is identified by one address (IP + port). That is not enough for a shared proxy for multi servers. Like HTTP HOST header, which enables virtual host in a web server machine [1]. The similar approach is used in Oracle net listener by adding INSTANCE_NAME [2]. Adding the host name field helps the proxy layer to find the backend server, monitor full throughput easily, and can do the specific logic on different host. For cloud service, let server instance can be identified by <server name, address (IP + port)> will help to simplified workflow for the frontend proxy.