Feedback: Backup and Restore Monitoring - invalid or undocumented reference to scheduled_backup_failed

Page: https://cockroachlabs.com/docs/v24.3/backup-and-restore-monitoring.html

## What is the reason for your feedback?

[ ] Missing the information I need

[ ] Too complicated

[ ] Out of date

[ ] Something is broken

[X] Other

## Additional details

The page makes a reference to `scheduled_backup_failed` in the following quote, but `scheduled_backup_failed` is not documented on this page. Perhaps it should say `schedules.BACKUP.failed` instead?

> Metrics are reported per node. Therefore, it is necessary to retrieve metrics from every node in the cluster. For example, if you are monitoring whether a backup fails, it is necessary to track `scheduled_backup_failed` on each node.

Jira issue: DOC-12308