Set up IMAP Prometheus exporter for monitoring Service Desk

Right now we are blind as to whether mail_room is doing anything for Service Desk or reply-by-email.

A quick thing we could do to improve this is to run an IMAP Prometheus Exporter, such as https://github.com/camptocamp/imap-mailbox-exporter. We'd need to fork it to export the Unseen messages in https://godoc.org/github.com/mxk/go-imap/imap#MailboxStatus, but that shouldn't be hard.

At the minimum, I might just run a local exporter and watch it during the GCP failover.