Render GPX Metadata as a single JSON structure.

def visit_Metadata(self, metadata, *args, **kwargs):
        """Render GPX Metadata as a single JSON structure."""
        result = OrderedDict()
        put_scalar = lambda name, json_name=None: self.optional_attribute_scalar(result, metadata, name, json_name)
        put_list = lambda name, json_name=None: self.optional_attribute_list(result, metadata, name, json_name)

        put_scalar('name')
        put_scalar('description')
        put_scalar('author')
        put_scalar('copyright')
        put_list('links')
        put_scalar('time')
        put_scalar('keywords')
        put_scalar('bounds')
        put_list('extensions')

        return result