Monitor repl slot size

Hello,
we used to monitor the replication slot size on postgres 9.6.6 with the
following query:

SELECT pg_xlog_location_diff(pg_current_xlog_location(), restart_lsn) FROM
pg_replication_slots WHERE slot_name = 'SLOT NAME';

We are moving to postgres 10.4 and we saw the pg_xlog_location_diff is not
there anymore.

I know we can re-create it following this link :
https://github.com/DataDog/integrations-core/issues/907

but, is there any better way to do it? Any replacement for that function on
postgres 10?

Thanks a lot,
Nicola