report: add cgroup metrics in a separate varlink service
Add CpuUsage, MemoryUsage, IOReadBytes, IOReadOperations, and
TasksCurrent in a standalone socket-activated varlink service.
The new systemd-report-cgroup service listens at
/run/systemd/report/io.systemd.CGroup and exposes:
- io.systemd.CGroup.CpuUsage
- io.systemd.CGroup.IOReadBytes
- io.systemd.CGroup.IOReadOperations
- io.systemd.CGroup.MemoryUsage (with type=current/available/peak)
- io.systemd.CGroup.TasksCurrent