Turns out 99,8 % of the log is about the socket connection
Looks like this is exacerbated by Wekan issue #1911, as it turns out 99,8 % of the log is about the socket connection (since 2018-09-15):
root@battra:~# grep -c -v "received client metadata from anonymous unix socket" /var/snap/wekan/common/mongodb.log
23426
root@battra:~# grep -c "received client metadata from anonymous unix socket" /var/snap/wekan/common/mongodb.log
10970660
Log rotation would still be nice, but nowhere near as critical if less than one percent of the current logging was produced (a few megabytes per year in my setup here, if I extrapolated correctly).