Last archived timestamp/xid for Point-in-Time Recovery

We archive the WAL to a remote storage and will perform Point-in-Time Recovery by replaying the log from that remote storage. Under heavy load, our archiving pipeline is not able to archive WAL quick enough and we would like to get more visibility on the lag.

pg_stat_archiver is great. However, it only reports the last WAL archived and the time when that WAL archived. Ideally, we would also like to get the last archived timestamp/xid corresponding to the recovery_target_time and recovery_target_xid in the recovery.conf setting [1]

Any ideas on how to achieve that?

[1] https://www.postgresql.org/docs/9.6/static/recovery-target-settings.html

Thanks

Tianzhou