Extending LSIF output with arbitrary metadata?

I maintain a debugger that I am adding more source navigation capabilities to. One issue that I've run across is that the view of the source as described in the LSIF and the view of the source as described by a compiler in the debug info do not always match. In my case, embedding information about the version of the compiler into the LSIF would be sufficient to solve this problem.

It's unclear to me whether LSIF is supposed to be considered generally extensible (i.e. people can tack on whatever additional properties they want to any entry) or whether nothing that is not in the specification is supposed to appear. If the latter, would there be an objection to adding a spot for arbitrary key/value pairs on the Metadata node? e.g., in vaguely idl-ish syntax

partial interface MetaData {
  additionalInfo?: map<string, string>,
}

Start by reviewing the LSIF Metadata node definition and the specification's rules for extensions. Determine whether arbitrary key/value metadata is currently permitted; done means resolving that question and, if it is not permitted, defining whether the Metadata node should gain an additionalInfo map.