Track apdex by "class" of room

[erikjohnston]

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+)

[clokep]

small rooms (<10 members), medium rooms (10 to 1000 members), large rooms (1000+), and maybe extra large rooms (10,000+)

Might also make sense to specifically have DMs (i.e. 2 members).

[MadLittleMods]

We did this for the /messages response time in #13545:

if member_count <= 2:
    return _RoomSize.DM_SIZE
elif member_count < 100:
    return _RoomSize.SMALL
elif member_count < 1000:
    return _RoomSize.SUBSTANTIAL
else:
    return _RoomSize.LARGE