How to pass metadata in POST request and then extract it?

I see, that RASA has created an abstract method get_metadata() inside InputChannel class.

Without creating custom connector, If I just want to inherit RestInput class and define the body only for get_metadata() method as given below.

class CustomRestInput(RestInput):
    def get_metadata(self, request: Request) -> Optional[Dict[Text, Any]]:
        return request.json.get("metadata", None)

How and where should I call get_metadata() method of CustomRestInputclass to get the metadata ?

Can you please suggest me on this.