Report cgroup metrics for better monitoring support

[simitt]

When APM Server runs inside a container cgroup stats should be collected for stack monitoring.

Related issues:
elastic/apm#291
Report cgroup stats if running in container
Support CgroupsV2 in processors and Metricbeat

[axw]

elastic/beats#21113 adds the cgroup metrics to libbeat. This is just reusing the existing cgroup support, and does not do anything towards cgroup v2 support.

[axw]

Opened elastic/kibana#79050 to use this in Stack Monitoring

[simitt]

@axw I see this is assigned to you, are you planning on testing this? All of the PRs in stack monitoring are merged and should be available with the next BC. If not - no worries, I have tested the changes through when they got merged.

[axw]

@simitt no I just never unassigned myself. Seeing as you already tested, I think we're good here.