journal: expose last 10 high priority logs as metrics (#42621)

This commit exposes the last 10 high priority logs as metrics so that
the systemd-report reports them. The entries are reported as
`io.systemd.Journal.HighPriorityMessage` and include all fields that are
printable as strings.

This is archived via a new socket-activated unit listens on
/run/systemd/report/io.systemd.Journal