Hi,
You can find group counter information in User manuals [1] and in this talk [2]
Specifically see https://ftp.osmocom.org/docs/latest/osmosgsn-usermanual.pdf "9.4 CDR configuration" and "14 Counters".
[1] https://ftp.osmocom.org/docs/latest [2] https://media.ccc.de/v/SE8HRK