pglogrepl_demo: how to handle with wal2json plugin output

So my questions is:
I wanna use `wal2json` instead of `pgoutput` or `test_decoding` output plugin, how to determine `clientXLogPos ` when application consume message succeess.
In your `pglogrepl_demo` example: you 're using `clientXLogPos = xld.WALStart + pglogrepl.LSN(len(xld.WALData))` with `pgoutput` plugin to determixe client xlogpos, right? If I use `wal2json`, how to determine that?
Can you help me for this
Thanks

Start with the pglogrepl_demo example and review how it computes clientXLogPos from pgoutput messages. Compare that behavior with wal2json output and determine what position should be acknowledged after the application consumes a message; document the resulting guidance for wal2json users.

Written by the indexing model from the issue text.