Preface: I work for New Relic =)
New Relic unfortunately doesn't officially support MongoDB... I was wondering if have you tried using 'rpm_contrib'?
'rpm_contrib' is community sourced instrumentation that is only authored by New Relic, but does contain instrumentation for MongoDB. It doesn't add quite the same level of depth as our official instrumentation, but often times gets the job done. It may be have you're looking for, as far as mongoid support.
https://github.com/newrelic/rpm_contrib
Please feel free to open up a ticket on our support site:
https://support.newrelic.com/
If you have any questions in the future! We're always happy to help.
Best,
Seve