mail: populate mailer counter when creating dryRun client (#4646)
mail: populate mailer counter when creating dryRun client (#4646)

If a dryRunClient is used it will cause panics when SendMail is called because the 
constructor doesn't populate the prometheus counter. This fix populates the counter.
bmw/boulder,letsencrypt/boulder,jcjones/boulder,jsha/boulder,bmw/boulder,jsha/boulder,jcjones/boulder,letsencrypt/boulder,jcjones/boulder,jsha/boulder,bmw/boulder,jcjones/boulder,letsencrypt/boulder