Provide list of all Synapse caches with default limits numbers and current sizes in bytes

[matrixbot]

This issue has been migrated from #8811.

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Description:
In homeserver.yaml we have caches section, where can fill cache factors - global, or per_cache.
But I can't find the way to lookup list of all cache names (best with short description) that use Synapse, with default values in numbers, and exact name for config file parameter.

For example, what is default number for get_users_who_share_room_with_user - 100, 1000, or 10000? If we will have the default numbers, we will can calculate result using simple math.

Other problem is cache name, for example, the *getEvent* cache must be named in yaml file as *getEvent*, getEvent, get_event, _get_event_ (with replacing * to _), or maybe event_cache_size - what is right version?

Also we need the way to lookup current cache size in bytes, not only in objects count, to understand which cache eats most of RAM, when we try to solve "Synapse eats all my RAM" problem.

P. S. And separate question about root parameter event_cache_size - is this limit for getEvent cache, and caches.global_factor value applies to it now? In yaml comment there are "Not affected by caches.global_factor." but I set it to 500K, and see in Prometheus that *getEvent* grows to 1600K with global_factor = 4.0!

Also in current homeserver.yaml template parameter event_cache_size located in root, not inside cache category, - is this right?

Regarding this change https://github.com/matrix-org/synapse/pull/6391/files/2619891343cc55b307ae36e6dda6247b2d6a5f73#diff-bfb2b1823a1f152109d0007dc9c8c6e83a99ea81ffa6edc08ecca5fc8b3475daR75 - event_cache_size variable in must be moved from the root to caches subcategory, right?