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 field as the new METRIC_FAMILY_TYPE_OBJECT.
Individual fields from a journal entry that are unprintable
(invalid utf-8) are skipped.
This is archived via a new socket-activated unit listens on
/run/systemd/report/io.systemd.Journal
