Metricbeat/K8s: "error getting group status: open /proc/<PID>/cgroup"

This error is not related to Metricbeat's system module, but to a new feature in Beats internal monitoring.

The system module is still understanding the -system.hostfs flag and reporting metrics correctly. The only effect of this error is that the internal monitoring metrics from Metricbeat are lacking the new cgroups data.

See this issue for more information: https://github.com/elastic/beats/issues/23267