Adding a username with a comma in it makes the entire cohort fail.
Real-life case: https://metrics.wmflabs.org/cohorts/#6935 contains https://commons.wikimedia.org/wiki/Special:Contributions/Бурачек,_Андрій_Всеволодович who has a comma in his username. The result of this cohort:
- without any possibility to remove this user and without marking this user as invalid.
As usernames with commas are allowed, they should be allowed by Wikimetrics as well.