Record/emit current memory usage per user

[lwatiker]

max_memory_usage_for_user allows configuring a max memory user per user and will reject a request if anticipated to put user over their max quota. However theres currently no way to know know much memory a certain user is using and if they're near the allocated limit.

It'd be great if Clickhouse could export memory memory per user as metric (or write to a systems table) so this value could be used in monitoring and alerting.

[filimonov]

select * from system.user_processes;

#50492