Class constants shouldn't be display in playground

[osigge]

If I have a (string) constant in my class e.g.

private static final TEST = "test";

this constant will show up in the playground. I expect it not to be shown.
Constants should be hidden by default, since you probably won't overwrite them. An alternative would be, to introduce a new property show on @ApiObjectField which toggles the inclusion on the playground (like in @ApiObject).