Find the ECS service usage for clusters. Calls
        :py:meth:`~._find_usage_one_cluster` for each cluster.

def _find_usage_clusters(self):
        """
        Find the ECS service usage for clusters. Calls
        :py:meth:`~._find_usage_one_cluster` for each cluster.
        """
        count = 0
        fargate_task_count = 0
        paginator = self.conn.get_paginator('list_clusters')
        for page in paginator.paginate():
            for cluster_arn in page['clusterArns']:
                count += 1
                resp = self.conn.describe_clusters(
                    clusters=[cluster_arn], include=['STATISTICS']
                )
                cluster = resp['clusters'][0]
                self.limits[
                    'Container Instances per Cluster'
                ]._add_current_usage(
                    cluster['registeredContainerInstancesCount'],
                    aws_type='AWS::ECS::ContainerInstance',
                    resource_id=cluster['clusterName']
                )
                self.limits['Services per Cluster']._add_current_usage(
                    cluster['activeServicesCount'],
                    aws_type='AWS::ECS::Service',
                    resource_id=cluster['clusterName']
                )
                # Note: 'statistics' is not always present in API responses,
                # even if requested. As far as I can tell, it's omitted if
                # a cluster has no Fargate tasks.
                for stat in cluster.get('statistics', []):
                    if stat['name'] != 'runningFargateTasksCount':
                        continue
                    logger.debug(
                        'Found %s Fargate tasks in cluster %s',
                        stat['value'], cluster_arn
                    )
                    fargate_task_count += int(stat['value'])
                self._find_usage_one_cluster(cluster['clusterName'])
        self.limits['Fargate Tasks']._add_current_usage(
            fargate_task_count, aws_type='AWS::ECS::Task'
        )
        self.limits['Clusters']._add_current_usage(
            count, aws_type='AWS::ECS::Cluster'
        )