Problem with current proxy

Ref - https://blog.openzeppelin.com/the-transparent-proxy-pattern

Problem with the current proxy implementation

You have to specify all functions upfront when the Proxy contract is being deployed.

contract StorageProxy is Ownable {
    uint public num;
    address implementation;

    constructor(address _implementation) Ownable(msg.sender) {
        num = 0;
        implementation = _implementation;
    }

    function setNum(uint _num) public {
        (bool success, ) = implementation.delegatecall(
            abi.encodeWithSignature("setNum(uint256)", _num)
        );
        require(success, "Error while delegating call");
    }

    function setImplementation(address _implementation) public onlyOwner {
        implementation = _implementation;
    }

}

Once this contract is deployed, there is no way for me to add a new getNum function in there.