Push rabbitmq metrics to Prometheus

There's a few rabbitmq metrics that haven't been pulled into prometheus but they shouldn't be too hard to add[9,10] (currently, we mostly use the real-time dashboard[11] anyway, so there wouldn't be any functionality regression).

[9] https://github.com/kbudde/rabbitmq_exporter [10] aha, no Debian package yet. [11] http://saatchi.internal.softwareheritage.org:15672/

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Migrated from T1361 (view on Phabricator)