Metric valueFont `fit` option should be applied uniformly across grid

[nickofthyme]

Is your feature request related to a problem? Please describe.

Currently the fit mode for valueFontSize applies individually to each element in the grid such that each grid value can use a different fontSize as shown below.

A better solution would be to find the limiting fontSize for all elements in the grid and apply is to all.

Suggested in elastic/kibana#186929 (comment)