[Lens][Docs] add metric coloring doc

[drewdaemon]

Summary

Fix #174379

Adds an FAQ section for the coloring behavior of the metric visualization

[github-actions]

Documentation preview:

• ✨ Changed pages

[elasticmachine]

Pinging @elastic/kibana-visualizations (Team:Visualizations)

[drewdaemon]

@amyjtechwriter please feel free to suggest corrections for any capitalization / style errors. Also, would be nice to get an understandability check from you. Thanks!

[stratoula]

Wonderful :)

I am not sure about the image with the arrow 🤔 Not sure who is the best person to ask from the design perspective.

@MichaelMarcialis do you know maybe?

[amyjtechwriter]

LGTM - I've left a few small suggestions. In terms of sense checking, the logic explanation makes sense to me so I hope our users agree! :)

[drewdaemon]

@elasticmachine merge upstream

[drewdaemon]

@MichaelMarcialis I'm going to merge this, but feel free to chime in after the fact.

[MichaelMarcialis]

I am not sure about the image with the arrow 🤔 Not sure who is the best person to ask from the design perspective.

@MichaelMarcialis do you know maybe?

Apologies for the delayed response. I'm uncertain if there is an existing designer(s) that is dedicated to the Elastic docs or what the current rules are for highlighting specific areas of an image. I imagine @KOTungseth may know the answer to both?