fix monitoring: show alerts only from first instance (#521)

We don't need to show alerts from longterm instance, because the alerts
have shorter timeout than metrics collection interval


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **New Features**
- Updated the `VMAlert` YAML template to generate only the first
`VMAlert` resource based on metrics storage values.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Andrei Kvapil <kvapss@gmail.com>

packages/extra/monitoring/templates/vm/vmalert.yaml

@@ -18,4 +18,5 @@ spec:
     url: http://vminsert-{{ .name }}.{{ $.Release.Namespace }}.svc:8480/insert/0/prometheus/api/v1/write
   resources: {}
   selectAllByDefault: true
+{{- break }}
 {{- end }}