The existing JSONSeqMonitor was saving/restoring the "origin"
when generating a new log-entry. This allows logging from
different origins (e.g. "org.osbuild.main") in a kind of
"out-of-band" fashion.
But this save/restore feels slightly inelegant because
JSONSeqMonitor feels like the wrong layer to deal with this.
This is why a new `with_origin()` helper is introduced that
will either reuse the existing context or create a new one
with the requested origin.
fd61bcdcab