code: def _find_usage_subnet_groups(self):
          
          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"],
                  )
docs: find usage for subnet groups