Before it was done in the same way than it is done in UTool, by
increasing an entry in the metadata file by X each time that X tweets
are added to that CSV. However, for a script that converts from Mongo to
CSV static collections that are not growing in size, it is just better
to just count the number of lines of each CSV file once that the
conversion process has ended.

This also supresses the risk of the metadata being corrupted due to bad
parallelization