install/0000_90_cluster-version-operator_02_servicemonitor: Info-level alert for available updates

The purpose of this alert is to give cluster-admins something they can
subscribe to (via Alertmanager notifications) to let them know when
they can initiate an update for a particular cluster.

It is info severity because we want the quick push notification, but
letting your cluster sit on the old version for a few days or a week
or so is unlikely to cause much trouble.  For things like CVE fixes, a
more timely update might be advisable, but the cluster-version
operator currently has no way to make the "critical CVE" vs. "no
fixes, but adds a feature" distinction.  So I'm punting on the
warn-level or so "CVO is getting grumpy about your failure to take the
long-recommended update" alert for now, and just addressing the
info-level "you could update, no pressure".

Author: wking

install/0000_90_cluster-version-operator_02_servicemonitor.yaml

@@ -51,6 +51,13 @@ spec:
         (time()-cluster_version_operator_update_retrieval_timestamp_seconds) >= 3600 and ignoring(condition, name, reason) cluster_operator_conditions{name="version", condition="RetrievedUpdates", endpoint="metrics", reason!="NoChannel"}
       labels:
         severity: critical
+    - alert: UpdateAvailable
+      annotations:
+        message: Your upstream update recommendation service recommends you update your cluster.  For more information refer to 'oc adm upgrade'{{ "{{ with $console_url := \"console_url\" | query }}{{ if ne (len (label \"url\" (first $console_url ) ) ) 0}} or {{ label \"url\" (first $console_url ) }}/settings/cluster/{{ end }}{{ end }}" }}.
+      expr: |
+        cluster_version_available_updates > 1
+      labels:
+        severity: info
   - name: cluster-operators
     rules:
     - alert: ClusterNotUpgradeable