[gen-metadata] More aggressively declare subsets in METADATA files

fonts.google.com launched a new specimen page last week, that reuses the improvements developed for the fonts.google.com/noto section specimen pages last year.

However, this has highlighted a problem with Kumbh Sans and Nabla, where they had Google Fonts Latin Plus glyph set support, but their METADATA files didn't declare the `math` subset, and therefore the fonts API stripped those characters from the fonts served to the specimen page, which was trying to show the full glyph set. The Kumbh Sans designer reported this (https://github.com/google/fonts/issues/5067) with a screenshot.

In https://github.com/google/fonts/pull/5095 I add the math subset to the 2 families we've noticed this issue surfaced, and I believe the root cause is that the METADATA generator has a threshold for when it auto-adds subsets to a METADATA file, such that when a font has only 1% of the math subset's characters, it won't add it but when 100% of latin-core subset is in the font, it is added. I forget the details on the threshhold, I think it was set per subset.

It seems that we probably ought to be more aggressive about declaring subsets, like if even 1 character from a subset exists in any font in the family, it should be declared in METADATA, and then using comments to include the % the tool found to inform later hand curation of the METADATA file.

We probably then need to make a special version of the METADATA generator to lint all the existing library and then PR a bunch of updates to a bunch of families.

It would also be good for the linter to tell us what unicode characters are in any font in a family that are not in any declared subset encoding, and not in any encoding (https://github.com/googlefonts/glyphsets/tree/main/Lib/glyphsets/encodings)