looking into the output folder of catboost, I see 3 types of metrics outputs:
tfevents (can be read by tensorboard) catboost_training.json (custom (?) format). Is read here to be shown as an ipython widget: https://github.com/catboost/catboost/blob/c2a6ed0cb85869a73a13d08bf8df8d17320f8215/catboost/python-package/catboost/widget/ipythonwidget.py#L93 learn_error.tsv, test_error.tsv, time_left.tsv which have the same data as json. Apparently they are to be used with this stale metrics viewer project: https://github.com/catboost/catboost-viewer