Track apdex by "class" of room

This issue has been migrated from [#13316](https://github.com/matrix-org/synapse/issues/13316).

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Occasionally we see send time issues in e.g. large rooms, which is not reflected in the global apdex (as the traffic isn't high enough proportion of the total request count).

To help surface these issues we could have a separate metric that separates the apdex by the "class" of room, e.g. small rooms (<10 members), medium rooms (10 to 1000 members), large rooms (1000+), and maybe extra large rooms (10,000+)