There is currently a known limitation with the MongoDB metrics available on FreeBSD. This means that some metrics like op counters and memory statistics will not be available if you are running MongoDB on FreeBSD. This will be indicated by errors in your Server Density agent log:

ERROR - getMongoDBStatus: memory KeyError exception - 'resident' 
ERROR - getMongoDBStatus: per second metrics KeyError exception - 'btree'

You can vote for improved statistics in MongoDB on FreeBSD.