Representative Run for User Flow

**Feature request summary**
We've been using Lighthouse Navigation for the past years as part of our CI flow.
As recommended to reduce variability we run LH 5 times and we use the [computeMedianRun](https://github.com/GoogleChrome/lighthouse/blob/main/core/lib/median-run.js#L47) function to retrieve the most representative report.

This works well for `navigation` mode but now we want to run User Flow tests and we don't know what is the best approach to find the most representative report. In `navigation` mode we use the normalized distance based on `FCP` and `TTI` but is it still relevant for other modes ? Multiple steps reports ?
* What is the recommended way to choose the most representative report ?
* Also why specifically `FCP` and `TTI` were the web vitals chosen to compute the median which serves as a base to retrieve the most representative report ?
* Which web vitals should we choose to compute the median for `timespan` mode ?
* How to handle the case of multiple steps ?