Enable failure notifications for Gitlay balancer cronjob

Gitlay balancer job is run on our ops instance once a day to rebalance gitlay shards based on disk space utilization. At the moment, it fails silently if something happens along the way. This is to enable notifications in case of job failures.

Desired outcome

• Add job metrics based on our notification framework: https://gitlab.com/gitlab-com/runbooks/-/blob/master/docs/uncategorized/job_completion.md
• Configure deadman snitch (DMS) for balancer cronjob to monitor failures to trigger job. Reference: https://gitlab.com/gitlab-com/runbooks/-/blob/master/docs/uncategorized/periodic_job_monitoring.md