gitlab: add function to get labels with counts

Labels API has recently changes and open/closed/merge requests count are not mandatory anymore

According to the documentation, there is now NO open_issues_count, closed_issues_count and open_merge_requests_count when trying to interrogate GET /projects/:id/labels.

It only appears when using GET /projects/:id/labels?with_counts=true.

This merge request:

• modify the label types to make those fields optional
• modify the label creation to make those fields to None
• create a new function labels_with_counts to get the counts (not the smartest or prettiest or most elegant way but I don’t know how to do this without breaking the API).

Topic-rename: label-counts-optional