Add tracking to see how big of a problem LL with limited sync responses are in practice

[bwindels]

add some tracking as described below:

so:
Matthew to add metrics on synapse to track how many sync responses are limited, and how many membership events they contain. I expect this to look like "incr sync for 40 rooms, 30 gaps, sent 243054 member events" in the synapse logs

meanwhile, we want to see just how slow incremental syncs are (in terms of the server processing & network bandwidth) time on the clients in general, for which we don't have metrics
i suggest we track on clients:

• time taken for initial /sync and incr /sync to return
• how many membership events there are, and how many rooms were limited?
• how long it takes to process the /sync result

in the short term, I'll disable LL on incremental syncs entirely, and so fix the bug like that
and then once we've gathered metrics from both server & clients, we can decide what the correct solution actually based on the data, where are options are:

• sync /members every time there's a gappy sync
• give up on LL for incre sync
• pass a since= param to /members (or use /sync) to keep the members list updated.

does that match everyone's understanding from the call just then?

[bwindels]

@ara4n is this still needed? I recall you mentioning the other day you already got some insight by logging this server-side?

[bwindels]

We're parking this for now, more important things to do