How to measure replication lag in logical replication setup.

HI,

How to measure replication lag time in case of logical replication?

this query is returning empty

SELECT now() - pg_last_xact_replay_timestamp() AS time_lag;
time_lag
----------

(1 row)

this is my slot details

SELECT * FROM pg_replication_slots;
slot_name | plugin | slot_type | datoid | database | active | xmin |
catalog_xmin | restart_lsn
-----------+--------+-----------+--------+----------+--------+------+--------------+-------------
slot_1 | | physical | | | t | 1983 |
| 0/50805C8

In my case slot_type is physical, does it look ok or for logical
replication the slot_type should be logical?

Thanks,
Ram