As of May 2011 the rpm_contrib gem has had instrumentation for the mongo ruby driver (last updated Feb 2013). rpm_contrib is being phased out in favour of separate instrumentation gems for each project, but as far as I can tell there is not yet a separate project for the mongo driver.
I have had some success with rpm_contrib on a staging app using mongomapper. Is there any reason I shouldn't add rpm_contrib to a production app?