alerts: Add alert to archive BURP disk

This alert will fire once the MD RAID resynchronization process has
completed and both disks in the array are online.  It will clear when
one disk is disconnected and moved to the safe.

group_vars/metricspi/alerts.yml

@@ -85,6 +85,17 @@ vmalert_rules:
           something happens to the active disk, such as hardware failure, power
           surge, fire, or accidental `rm -rf`, the offline disk is only out of
           date by a few weeks.
+    - alert: disk needs archived
+      expr:
+        collectd_md_md_disks{instance="burp1.pyrocufflink.blue", type="missing"} < 1
+      annotations:
+        summary: One of the disks in the BURP array should be archived
+        description: >-
+          The disks in the BURP RAID-1 (mirror) array should be swapped
+          periodically.  One disk should be online and mounted while the other
+          is stored in the fireproof safe.  All of the disks are currently
+          online; one needs to be disconnected and moved to the safe as soon as
+          possible.
 
   - name: certificates
     rules: