enable backups on metricsdb-01 before going to production

in #41372 (closed), we decided to just disable the postgresql backups on metricsdb-01 during development because it was causing too much churn on the backup server (3TB/week). we need to figure out what's causing this and reduce it to a point where we can again use the backup server.

we absolutely should re-enable backups before we move this server in production.

/cc @hiro let me know if there's a better place to file this bug report. and could we set a due date here?