Don't fail if wpt-metadata has unrecognised entries

[jgraham]

In #2134 we added support for labels in metadata, after not noticing that it had been broken for quite some time.

In practice all we care about is updating bugs, so maybe we should parse any kind of metadata that doesn't match some more specific form into a MetaEntryGeneric which would just store whatever fields exist in a dict. That might avoid a situation where the format changes slightly in the future, we forget to update this code, and so we get silent failures trying to notify for syncs.