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

Hi pg-hackers,

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