Loading meta-data from JSON silently drops proto2 extensions

[hatyo]

The yaml-tests framework can load a schema template from a JSON meta-data file (load schema template ... from <file>.json, implemented in CommandUtil.loadRecordMetaDataFromJson). That parse goes through JsonFormat, which implements the proto3 JSON mapping and treats proto2 extensions as if they did not exist. Extensions written into the file are therefore dropped on the way back in, without any warning.

This matters because descriptor options that the meta-data depends on are carried by extensions. A vector field is a bytes field whose precision and dimensions live in an extension of google.protobuf.FieldOptions:

  "embedding": { "type": "TYPE_BYTES",
    "options": { "com.apple.foundationdb.record.field": {
        "vectorOptions": { "precision": 64, "dimensions": 512 }}}}

After loading, that field is a plain bytes column instead of a vector