Agreed, we might want to add this too.
Until then you could pass the options to convert() and then get them from the Document:

        asciidoctor.convert(document, OptionsBuilder.options()
                .option('credentials-path', '/path/to/credentials.json'').backend('googleslides')

And then in the Converter:

    @Override
    String convert(ContentNode node, String transform, Map<Object, Object> opts) {
      String credentials = node.getDocument().getOptions().get('credentials-path')
      ...
    }

I think this solution would also have some advantages in multithreaded environments when different threads/contexts would require different credentials.

Thanks Robert for your reply.

Another alternative would be to retrieve the Document (and therefore the options) from the constructor. I didn't try but reading the Ruby code I believe that Asciidoctor (Ruby) passes the Document as an options when the converter is instantiated/created (see lib/asciidoctor/document.rb#L1128-L1144)

So in theory, the following should work:

class GoogleSlidesConverter extends AbstractConverter {
  public GoogleSlidesConverter(String backend, Map<Object, Object> opts) {
    String credentials = ((Document) opts.get("document")).getOptions().get("credentials-path")
  }
}

Yes, that should work too.