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 ?