☂️ Rethink opportunity reporting

**Summary**

I was trying to find the issue for where we've discussed surfacing opportunities *per metric* and I couldn't 😮 given Fraggle Rock, lots of recent suggestions for the opportunities in misc Google Docs, emails, etc, I thought we should have a place to track all the suggestions.

Non-exhaustive list of things we might want to do with opportunities in the future that we'll need to think about during design

- report the same audit's results on different metrics, e.g. minifying your scripts might save you a bit on LCP and a lot on TTI or fixing these images will save you lot on download time but not help any of your metrics
- very related: surface opportunity savings in general terms with sparklines and ranges (fraggle rock)
- highlight important LCP savings of particular details items, i.e. this image is your LCP image so optimize that for huge LCP savings but the rest aren't as important
- surface the performance score if all opportunities were done (https://github.com/GoogleChrome/lighthouse/issues/8740)
- report some idea of optimization difficulty (https://github.com/GoogleChrome/lighthouse/issues/10428)