collect client-side telemetry (useChat)

[lgrammel]

Feature Description

I think we should start instrumenting 2 metrics that map to TTFT and TOK/s, but the trigger when we perform UI transitions on the client.

Submit starts timer
When we receive the first token in the frontend that causes us to transition into a first token UI state we mark: TTFUI
And then whenever we actually change the UI display we tick a unit for TOKUI/s

Such a metric would map to actual UX, it would catch regressions through egregious buffering at any layer

Use Cases

No response

Additional context

No response