Extend io.systemd.CGroup.CpuUsage from a single per-unit nanosecond counter to three rows distinguished by a "type" field of "total", "user", or "system". The values come from cpu.stat's usage_usec, user_usec and system_usec keys, read in a single keyed-attribute fetch and cached on each CGroupInfo so each scrape only opens cpu.stat once per cgroup.