[Metric] allow array of values

[markov00]

Add the possibility to render an array of values in a concatenated way.

When passing an array of values a bar can't be rendered and the background color could only have a categorical meaning because is not possible to link a single color to a set of values.

Screenshots (if relevant):

This is how the legacy metric will render concatenated strings:

Kibana cross issue
elastic/kibana#160815

[markov00]

cc @gvnmagni

[gvnmagni]

What do you all think about using a lighter font-weight for characters that are not part of values, as we can see in the left option where brackets and commas are in regular font weight instead of bold