[Lens] Allow palette configuration for metric

[flash1293]

There is a nice palette configuration UI for table coloring, but metric visualizations can't be colored based on the value. We should expose the same palette configuration UI to allow the user to set the coloring scheme in a way they are accustomed to.

The UI has to be restricted in some ways and defaults have to be found, because as there is just a single value it's not possible to default to "percentage of range"-based coloring as no range can be calculated.

A similar problem has to be solved for gauge visualizations

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[ghudgins]

for metric we need to evaluate which features we keep:

• should we do color continuity (maybe....)
• default: 0-100 (easy, what visualize does) or try to infer a smarter default from the metric
• don't do percentage mode as there are no ranges

[ghudgins]

additional question from planning week: what do we do if the user changes palettes

[ghudgins]

Nice to have: affordance when the aggregation is Percentile https://discuss.elastic.co/t/change-colour-of-metric-visualization-for-percentile-ranks-aggregation/277454/4

[flash1293]

When doing this we should make sure the expression / renderer integration will be easy to integrate with #114116