find usage for NAT Gateways

        :param subnet_to_az: dict mapping subnet ID to AZ
        :type subnet_to_az: dict

def _find_usage_nat_gateways(self, subnet_to_az):
        """
        find usage for NAT Gateways

        :param subnet_to_az: dict mapping subnet ID to AZ
        :type subnet_to_az: dict
        """
        # currently, some regions (sa-east-1 as one example) don't have NAT
        # Gateway service; they return an AuthError,
        # "This request has been administratively disabled."
        try:
            gws_per_az = defaultdict(int)
            for gw in paginate_dict(self.conn.describe_nat_gateways,
                                    alc_marker_path=['NextToken'],
                                    alc_data_path=['NatGateways'],
                                    alc_marker_param='NextToken'
                                    )['NatGateways']:
                if gw['State'] not in ['pending', 'available']:
                    logger.debug(
                        'Skipping NAT Gateway %s in state: %s',
                        gw['NatGatewayId'], gw['State']
                    )
                    continue
                if gw['SubnetId'] not in subnet_to_az:
                    logger.error('ERROR: NAT Gateway %s in SubnetId %s, but '
                                 'SubnetId not found in subnet_to_az; Gateway '
                                 'cannot be counted!', gw['NatGatewayId'],
                                 gw['SubnetId'])
                    continue
                gws_per_az[subnet_to_az[gw['SubnetId']]] += 1
            for az in sorted(gws_per_az.keys()):
                self.limits['NAT Gateways per AZ']._add_current_usage(
                    gws_per_az[az],
                    resource_id=az,
                    aws_type='AWS::EC2::NatGateway'
                )
        except ClientError:
            logger.error('Caught exception when trying to list NAT Gateways; '
                         'perhaps NAT service does not exist in this region?',
                         exc_info=1)