[Lens] Add coloured arrows representing up/down trends to Metrics

[teresaalvarezsoler]

Describe the feature:
Currently, it is possible to calculate changes in Metrics using the "shift" command in the formula.

The problem is the format, users would like to visualize this change using colours (green/red) and arrows (up/down). Something like this (needs proper design)

[elasticmachine]

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

[markov00]

cc @gvnmagni I believe we already have a design for that using the icon in the top right corner of the screen, we can double-check if this is aligned with the existing requirements here.

[gvnmagni]

What we can offer right now I believe is to use the icon at the top to show increase/decrease in values. Theoretically at least, I have no idea of what does that mean in term of implementation but I am pretty sure that there will be a little bit of work to be include this functionality.

If we want to go in this direction, which I like, I would suggest to make it even better and get closer to what Teresa is suggesting, including directly within the extra-value this kind of information which I believe is much more readable and compact.

Happy to work on this if it's a priority!