Add appearance settings for metric chart

[alaudazzi]

This PR adds the new appearance settings for the metric chart.

Closes #3088
Relates to elastic/kibana#229815 and elastic/kibana#233992

[github-actions]

🔍 Preview links for changed docs

• explore-analyze/visualize/lens.md

[alaudazzi]

@mariairiartef
Do you think we should be more explicit about the fact that the Secondary metric alignment and the icon position are enabled only when a secondary metric gets created? It seems obvious, but let me know what you think.

CC @gvnmagni

[mariairiartef]

Why not calling this section "Metric chart"? In other places in the same page they are called like this, such as "Show trends in Metric charts"

[mariairiartef]

I saw that it is also called metric visualization in "How does dynamic coloring work for the metric visualization?"

[mariairiartef]

@mariairiartef Do you think we should be more explicit about the fact that the Secondary metric alignment and the icon position are enabled only when a secondary metric gets created? It seems obvious, but let me know what you think.

CC @gvnmagni

I'd suggest we follow the approach used in the docs for consistency

[gvnmagni]

Here I am, thank you Arianna for the effort. I believe the only note I have is that where we describe the possibilities for primary metric we say:

Primary metric
Define the orientation of the primary metric in terms of Position, Alignment, and Font size.

The only concern I have is around the term Orientation, that I am not sure brings the right concept. I would have chosen a word like "placement" but I am not a native speaker and I might be have a misleading idea of the term Orientation in mind. Happy to keep it if it is instead more suited

[alaudazzi]

@gvnmagni
Agreed, good point. On a second thought, as we are defining the Font size other than the Position and Alignment, we should probably rephrase with:

Primary metric
Define the formatting of the primary metric in terms of Position, Alignment, and Font size.

[gvnmagni]

That would work perfectly!

[florent-leborgne]

LGTM!