please provide authoritative source for METADATA.pb protobuf schema

[demonti]

I am trying to read the METADATA.pb files from the google/fonts repository, but fail as the schema files (fonts_public.proto) I found in the googlefonts repositories seem to be outdated. Specifically, some fonts (Monserat, Merriweather, Oswald, Lato, OpenSans) contain the target_type "TARGET_OS_ANDROID", but no schema (e.g. in the fontbakery or gftools repositories) contains this enumeration value. There may be further discrepancies, but I have not dived into it yet.

So there should be an authoritative source for the schema file, maybe even in the gf-docs repository, and it should be up-to-date with the METADATA.pb files occurring the the google/fonts repository. The documentation should contain a link to that file.