feat: add Prometheus pushgateway and metric collection for OVN backup

[awfabian-rs]

JIRA:OSPC-551

[awfabian-rs]

I tested this on SJC. This seems merge-able to me right now, but I still want to write an alert based on the collected metrics so we know if OVN backup doesn't complete, and perhaps add a grafana dashboard for the metrics, so I've marked it as a draft.

[awfabian-rs]

• Un-marking as draft. Please review and merge if appropriate or indicated needed changes.
• This should collect metrics from the OVN backup CronJob and alert if they don't upload to Swift.
  • It needed a pushgateway to collect metrics, so the PR includes adding a pushgateway.
  • It also looks for >= 80% usage on the on-cluster volume storing the OVN backups.
• I tested this on my lab and SJC (in an isolated namespace)
  • I tested on SJC in an isolated namespace because we don't have the Swift tempauth used for saving the OVN backups off-cluster in any other environment yet
  • Prometheus discovers and scrapes the pushgateway from the k8s ServiceMonitor object
    • and thereafter, I can retrieve the metrics.
  • The PrometheusRule object created for the alertmanager rules passes validation
    • i.e. prometheus-operator-validated: "true" on the PrometheusRule YAML output.
• A grafana dashboard doesn't seem too essential here
  • I needed the data to alert on, not necessarily a dashboard
  • It seems easier to add later once Prometheus has collected some data to display if we want to add one.
    • so I skipped this for now