Report telemetry on client health

[ara4n]

As per convo on #riot-dev, I'm wondering if it would be worthwhile to track client health telemetry and feed it into Chrome Dev Tools like react-perf-tools (or some browser-agnostic client-side performance tracking library).

Specifically:

• Frequency of dispatches (to track whether we unintentionally do something like https://github.com/matrix-org/matrix-react-sdk/commit/c665c1170 which massively increases the number of actions flying around)?
• Number of listeners? (to see how much those actions are fanning out)
• Maximum (or average?) time taken to process dispatches?

[lampholder]

• memory profile (v8 heap size, native (RSZ) size, how much time spent major/minor GCing? - if we can get at any of that)
• time to switch room - https://github.com/matrix-org/matrix-react-sdk/pull/1814
• time to reorder the RoomList?
• and maybe other common regression areas
• unhandled exceptions (especially UISI root causes)

(list expanded out by @ara4n)

[ara4n]

Further to the discussion of "how much would we win if we lazyloaded room members", some other revealing metrics could be:

• Jank? (i.e. maximum time taken to render a frame; if we can get that programmatically)
• Hardware stats on the browser (so we can normalise the telemetry to see if it's just a slow/overloaded/broken computer rather than our shitty code - this might be as easy as calculating a bogomips style benchmark during startup, which averages out over time by being stored in local storage. We used this very effectively in the TNG days to determine which video capabilities a given device should advertise for video calling.)
• number of rooms, number of state events, number of members, number of timeline events
• number of roomtiles currently in the RoomPanel? (i.e. "is this horrible jank caused by Matthew expanding all his RoomSubLists")
• time taken to login for the first time on a device (until the app is usable)
• time taken to open Riot (having previously logged in) (until the app is usable)

[lampholder]

Some of these have been pulled out as individual items:

• https://github.com/vector-im/riot-web/issues/6322 - time to load app
• https://github.com/vector-im/riot-web/issues/6323 - indexeddb usage
• https://github.com/vector-im/riot-web/issues/6324 - heap size
• https://github.com/vector-im/riot-web/issues/6325 - time to change rooms
• https://github.com/vector-im/riot-web/issues/6390 - number of UISIs

[lampholder]

Removing this from the milestone since it's the achievable, expected to close chunks that should be in the milestone (rather than the overarching 'epic')