find usage for subnet groups

def _find_usage_subnet_groups(self):
        """find usage for subnet groups"""
        paginator = self.conn.get_paginator('describe_db_subnet_groups')
        for page in paginator.paginate():
            for group in page['DBSubnetGroups']:
                self.limits['Subnets per Subnet Group']._add_current_usage(
                    len(group['Subnets']),
                    aws_type='AWS::RDS::DBSubnetGroup',
                    resource_id=group["DBSubnetGroupName"],
                )