hosts/clerie-backup: Export metrics for backup replication to Hetzner

hosts/clerie-backup/configuration.nix

@@ -52,6 +52,11 @@
       environment = { BORG_RSH = "ssh -p 23 -i /var/src/secrets/ssh/borg-backup-replication-hetzner"; };
       compression = "auto,lzma";
       startAt = "*-*-* 04:07:00";
+
+      readWritePaths = [ "/var/lib/prometheus-node-exporter/textfiles" ];
+      postPrune = ''
+        echo "backup_replication_hetzner_last_successful_run_time $(date +%s)" > /var/lib/prometheus-node-exporter/textfiles/backup-replication-hetzner.prom
+      '';
     };
   };