track CPU by users and log a scoreboard occasionally?

This issue has been migrated from [#9169](https://github.com/matrix-org/synapse/issues/9169).

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much as we log a scoreboard of the top rooms contributing to stateres every 2 minutes, i suspect it would be useful to do so too for per-user accounting (rather than me having to do horrible perl to calculate one on the fly by scraping the logs), so you can see which users are responsible for chewing all your CPU on a given process or worker.