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.