Expose Prometheus metrics from etcd-backup-operator

[cpick]

Expose a HTTP /metrics endpoint from etc-backup-operator to serve Prometheus metrics.

Currently "etcd_backup_operator_controller_backups_created" and "etcd_backup_operator_controller_backups_failed" record the number of backups that succeeded and failed, respectively.

These allow alerting/monitoring based on the health of backups.

[etcd-bot]

Can one of the admins verify this patch?

[etcd-bot]

Can one of the admins verify this patch?

[etcd-bot]

Can one of the admins verify this patch?

[caarlos0]

this would be awesome!