The rsyslog container is needed by the MetMon DFG as part of the deprecation and elimination of fluentd for OSP 16.
IN OSP16 fluentd composable service is replaced by rsyslog composable service. For that we need [1] to be pulled downstream. [1] https://github.com/openstack/kolla/blob/master/docker/rsyslog/rsyslog/Dockerfile.j2