In this case, I usually use "mysqltuner.pl" (http://mysqltuner.pl,
major/MySQLTuner-perl) script that helps to better understand what maximum memory usage is possible (and what else could be tuned).
As it is a Plesk forum with community, there are my personal suggestions that could help to investigate the problem by yourself. I am interested in improving Advanced Monitoring and monitor any interesting cases and questions about it
I sure Vladimir will contact you to resolve the ticket because it is a separate workflow from the forum.
Unfortunately the issue is still there and MySQL memory usage continues to grow (now is over 2.2GB... 100MB every single day with no excessive traffic/use on the websites):

Your payment technical support give me the suggestion to use mysqltuner (they don't offer assistance about configuring and adjusting MySQL settings)... so, could you explain me the instructions "for dummies" to install this script?
Is safe to install it on a production server?
When I'll have finished to monitoring is necessary uninstall it?
Thanks.