meta model subscriber on resource registration.

        We watch for PHD event that provides affected entities and register
        the health-event filter to the resources.

def register_resources(klass, registry, resource_class):
        """ meta model subscriber on resource registration.

        We watch for PHD event that provides affected entities and register
        the health-event filter to the resources.
        """
        services = {'acm-certificate', 'directconnect', 'dms-instance', 'directory', 'ec2',
                    'dynamodb-table', 'cache-cluster', 'efs', 'app-elb', 'elb', 'emr', 'rds',
                    'storage-gateway'}
        if resource_class.type in services:
            resource_class.filter_registry.register('health-event', klass)