v-m/alerts: Fix PostgreSQL WAL archive failed alert

The `pg_stat_archiver_failed_count` metric is a counter, so once a WAL
archival has failed, it will increase and never return to `0`.  To
ensure the alert is resolved once the WAL archival process recovers, we
need to use the `increase` function to turn it into a gauge.  Finally,
we aggregate that gauge with `max_over_time` to keep the alert from
flapping if the WAL archive occurs less frequently than the scrape
interval.

victoria-metrics/alerts.yml

@@ -185,7 +185,9 @@ groups:
     for: 10m
   - alert: WAL archive process failed
     expr: >-
-      pg_stat_archiver_failed_count > 0
+      max_over_time(
+        increase(pg_stat_archiver_failed_count)[20m]
+      )> 0
     annotations:
       summary: The archiver process failed for one or more WAL segments
       description: >-