Request for metrics that are inclusive to Assistive Technology

**Feature request summary**
Per @addyosmani's request, I'm following up with a feature request after today's discussion here, https://twitter.com/scottjehl/status/1278372113716576258 which duplicates Léonie Watson's [earlier tweet](https://twitter.com/LeonieWatson/status/1260270620518035456)

It would be useful to know when Assistive Tech is able to interact with and communicate page content so that that timing can be factored into existing metrics that represent page "readiness". Measurements that note or approximate the time when the accessibility tree is built would be interesting to see and understand, and as Léonie's tweet mentions, the time of the first accessibility API query as well.

It'd also be interesting to know which existing metrics are not relevant to Assistive Tech (for example, is FCP happening before the accessibility tree is created due to blocking JS?). Or, in an SSR scenario, is the accessibility tree created initially one way and later "hydrated" into a much different state?

**What is the motivation or use case for changing this?**

To see metrics that measure when a page is usable to all users, including those using AT.

**How is this beneficial to Lighthouse?**

Accessibility is part of Lighthouse scoring criteria.