Metadata: add description

def metadata_add_description(self):
        """ Metadata: add description """
        service_description = {}
        if (self.args.json):
            service_description = json.loads(self.args.json)
        if (self.args.url):
            if "url" in service_description:
                raise Exception("json service description already contains url field")
            service_description["url"] = self.args.url
        if (self.args.description):
            if "description" in service_description:
                raise Exception("json service description already contains description field")
            service_description["description"] = self.args.description
        metadata = load_mpe_service_metadata(self.args.metadata_file)
        # merge with old service_description if necessary
        if ("service_description" in metadata):
            service_description = {**metadata["service_description"], **service_description}
        metadata.set_simple_field("service_description", service_description)
        metadata.save_pretty(self.args.metadata_file)