Add velero_backup_warning_total metric

[varac]

Describe the problem/challenge you have

The current prometheus exporter doesn't alert when there are warnings in backup sets. I.e. this backup had a warning because it could not find the wordpress-wp-uploads volume, because it was renamed (rather serious i.m.o.):

❯ velero get backup                                            
NAME                          STATUS      ERRORS   WARNINGS   CREATED                          EXPIRES   STORAGE LOCATION   SELECTOR
velero-daily-20220908033056   Completed   0        1          2022-09-08 05:30:56 +0200 CEST   14d       default            <none>
...

❯ velero backup logs velero-daily-20220908033056 | grep -i warn
time="2022-09-08T03:31:36Z" level=warning msg="No volume named wordpress-wp-uploads found in pod stackspin-apps/wordpress-0, skipping" backup=velero/velero-daily-20220908033056 logSource="pkg/restic/backupper.go:143" name=wordpress-0 namespace=stackspin-apps resource=pods

But the velero exporter still shows velero_backup_failure_total as 0.

Describe the solution you'd like

I'd like to propose an additional metric called velero_backup_warning_total.

Environment:

• Velero version (use velero version) : Client: v1.9.1, Server: Version: v1.9.1
• Kubernetes version (use kubectl version): v1.24.4
• Kubernetes installer & version: k3s v1.24.4+k3s1
• OS (e.g. from /etc/os-release): Server: Debian bullseye

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