Please merge PR: https://github.com/percona/mongodb_exporter/pull/58.
This low-priority/nice-to-have addition to exported metrics and it this NOT blocking anything. This adds stats on replication execution, available since MongoDB 3.0. More info here: https://docs.mongodb.com/manual/reference/command/serverStatus/#serverstatus.metrics.repl
No grafana changes necessary, I would like to add the metrics to the exporter only, then graph this data in a later release.