This checks monitors time how long MongoDB needs
to write the data to disk. You can configure levels for the
average flush time and the last_flush time. Without further
configuration this check is always OK.
Note: If the MongoDB is running as a replica set, this check is not discovered
on the same host where the
mk_mongodb plugin is installed. The data is piggybacked
to a pseudo host named after the replica set name, intead.
One service is created.