Currently, Metricbeat monitors MongoDB instances using Mongo's serverStatus() report. This is the primary "data dump" of information regarding a particular Mongo instance.
In addition, Mongo has other useful reporting tools that I think the broader user base would like to leverage. For starters, I think it would be great if Metricbeat is optionally capable of reporting on db.stats().
DB.stats() provides more granular insight into the host's resources utilized by a particular Mongo database. The fields are as follows:
Db.stats() metrics are important for evaluating the host's resource usage for a particular database, and is very useful in sharding decisions/architectures.
If you're interested in the conversation, please refer to Mongobeat's initial pull request which discusses db.stats() in part.