v-m: alertmanager: Group disk usage alerts

Some machines have the same volume mounted multiple times (e.g.
container hosts, BURP).  Alerts will fire for all of these
simultaneously when the filesystem usage passes the threshold.  To avoid
getting spammed with a bunch of messages about the same filesystem,
we'll group alerts from the same machine.

victoria-metrics/alertmanager.config.yml

@@ -17,6 +17,11 @@ route:
   - '...'
   receiver: ntfy
   routes:
+  - receiver: ntfy
+    matchers:
+    - alertname=DiskUsage
+    group_by:
+    - instance
   - receiver: ntfy
     matchers:
     - alertgroup=Frigate