Prometheus metric for successful mails or delivery errors

There was a recent issue with our email provider and unfortunately we were unable to alert on the issue due to lack of monitoring. We did receive an email from our mail provider but it would have been better to alert and notify our customers much sooner.

See https://sentry.gitlap.com/gitlab/gitlabcom/issues/218714/ for error that was generated. We also saw this in logging https://log.gitlab.net/goto/bb2f2c7e24090a83a343bd3f95985017

In this case it would be very helpful to have a counter for successful emails, this would allow us to alert when the email send rate falls below a certain value.