restic-exporter: Deploy Restic Prometheus exporter

The [restic-exporter][0] exposes metrics about Restic snapshots as
Prometheus metrics.  This allows us to get similar data as we have for
BURP backups.  Chiefly important among the metrics are last backup time
and size, which we can use to determine if backups are working
correctly.

[0]: https://github.com/ngosang/restic-exporter

restic-exporter/restic-exporter.env

@@ -0,0 +1,4 @@
+TZ=America/Chicago
+RESTIC_REPOSITORY=s3:https://burp.pyrocufflink.blue:9000/restic
+INCLUDE_PATHS=True
+REFRESH_INTERVAL=3600