Mailgun exporter not moved to Mimir.
This was set up in production-engineering#24973 (closed) to export things to prometheus, and does not exist in Mimir.
Compare https://dashboards.gitlab.net/d/mailgun-main/mailgun3a-overview?orgId=1 to https://dashboards.gitlab.net/d/mailgun-main/mailgun3a-overview?orgId=1&var-PROMETHEUS_DS=PA258B30F88C30650.
This should be addressed before we turn off the old prometheus hosts in production#18216 (closed).