Specify `is_parametric` in axis metadata

[nathan-williams]

Describe the issue

The pipeline is prepared to accept axis metadata with is_parametric.

optional bool is_parametric

We should add the corresponding field to the upstream proto definition and populate the field for all parametric axes. There are currently 9 as I understand it: x_opaque, x_transparent, x_transparent_figures, y_opaque, y_transparent_ascender, y_transparent_descender, y_transparent_figures, y_transparent_lowercase, and y_transparent_uppercase.

[vv-monsalve]

Hi, @nathan-williams. Could you help us gain more context about this new field? For example, what is the purpose and effect of adding it?

[nathan-williams]

The only known use today is to inform axis ordering. For example, in a list of axes, we would want to show GRAD before XOPQ. This property has existed on our side for quite a while, but we realized just recently that the property was not being populated.

Let me know if I can support in identifying the parametric axes. They stand out pretty well among the existing axes if you know what to look for (i.e. axes with tags that match /(X|Y).../ more or less).

@davelab6 may be able to provide more insight into the definition of a parametric axis.