ContainerID support for non docker containers

[ndubss]

Add_docker_metadata needs to be refactored to get containerID of non docker containers.
https://github.com/elastic/beats/blob/ce68c7814e5c34ff417701481e1f65f1d5e100cf/libbeat/processors/add_docker_metadata/add_docker_metadata.go#L257-L264

Lookup CID using process cgroup membership data can be moved to add_process_metadata
https://github.com/elastic/beats/blob/ce68c7814e5c34ff417701481e1f65f1d5e100cf/libbeat/processors/add_docker_metadata/add_docker_metadata.go#L142-L148

@vjsamuel add anything you think I've missed

[andresrc]

Can you provide some more detail on the ask? E.g, which runtimes are you expecting to be supported? Thanks!

[vjsamuel]

Containerd is one such use case. Under the hood it just uses systemd to create cgroups and can be seen as systemd.slice in the cgroup files.