How to check stream replication latest history status

Hi All

I set up Postgres 11 stream replication env.Use keepalived to control
VIP and a script to check Postgres status.
Usually. I could check the stream replication status by this view
"pg_stat_replcation" on primary server.
Check this view "pg_stat_wal_receiver" on standby server.

Sometime the primary server abnormal or the network interrupt by
unexpected issue.

Both of them no record "pg_stat_replcation", "pg_stat_wal_receiver"
on that time.

I want to know the stream replication gap between primary and standby
before standby promote.
If the primary and standby have long replay_lag. I don't want to
standby server promote.

Is there any view or function which store the latest stream
replication gap(replay_lag) between primary and standby ?
Or it there any workaround to know this stream replication
gap(replay_lag) infomation ? Thanks for your help.

Jack.Xu