alerts: Add alert for BURP RAID array swap

This alert counts how long its been since the number of "active" disks
in the RAID array on the BURP server has changed.  The assumption is
that the number will typically be `1`, but it will be `2` when the
second disk synchronized before the swap occurs.

group_vars/metricspi/alerts.yml

@@ -46,3 +46,23 @@ vmalert_rules:
       expr: collectd_md_md_disks{type="missing", instance!~"burp.*"} != 0
     - alert: mdraid failed disk
       expr: collectd_md_md_disks{type="failed"} != 0
+
+  - name: BURP RAID
+    rules:
+    - alert: disks need swapped
+      expr:
+        time() - tlast_change_over_time(
+          (
+            collectd_md_md_disks{instance="burp1.pyrocufflink.blue", type="active"}
+            or last_over_time(collectd_md_md_disks{instance="burp1.pyrocufflink.blue", type="active"})[1d]
+          )[1d]
+        ) > 86400 * 30
+      annotations:
+        summary: The disks in the BURP array need swapped
+        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.  Switching them ensures that even if
+          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.