Monitoring: Tahoe CAs via Loki: Try to get timestamps from filenames

I hoped if I set correct timestamps from the RFC3339-like one
we have in the CAs' filenames, the problem of Loki being somehow
overwhelmed and presenting me all and every tahoe corruption advisory
for any time I ask would maybe go away.
But I can't even get the timestamp parsing to run, and Loki misbehaves
in so many ways, I'll just drop the effort of scraping tahoe CAs
with Promtail/Loki alltogether.