Metrics for monitoring master-slave replication
In the primary side, you have to use the view pg_stat_replication and compare differents fields to see the drift. The function pg_xlog_location_diff give the drift in size : 

select   pid, client_addr, state, sync_state,  
         pg_xlog_location_diff(sent_location, write_location) as write_lag,  
         pg_xlog_location_diff(sent_location, flush_location) as flush_lag,  
         pg_xlog_location_diff(sent_location, replay_location) as replay_lag
from pg_stat_replication ;


You can use the lag in monitoring tools, as gauge, to monitor your replication. 

If a standby is missing, you have to check in the log's standby why. 

If you use replication slots, you have to check the pg_replication_slots view.