[Design] RUM Core vitals + Key Metrics

[andrewvc]

We'll need to add the following metrics to the RUM page (all averages):

1. CLS: Cumulative Layout Shift
2. FID: First Input Delay
3. TBT: Total Blocking Time
4. LCP: Largest Contentful Paint
5. FCP: First Contentful Paint

• We can put them in one UI block, though we may later group CLS,LCP and FID under Core vitals.
• These values all have their own ranges of good to warning to bad, and should probably change color from green -> yellow -> red. Each metric has its own thresholds for those categories (we will use the mobile guidelines from google for these thresholds which are more permissive about timings).
• We need a note somewhere in the UI that these are only available in chromium based browsers.

[elasticmachine]

Pinging @elastic/observability-design (design)

[andrewvc]

Closing as dupe of https://github.com/elastic/uptime/issues/233