Because logical replication is based on a similar architecture as physical streaming replication, the monitoring on a publication node is similar to monitoring of a physical replication primary (see Section 26.2.5.2).
   The monitoring information about subscription is visible in
   
   pg_stat_subscription.
   This view contains one row for every subscription worker.  A subscription
   can have zero or more active subscription workers depending on its state.
  
   Normally, there is a single apply process running for an enabled
   subscription.  A disabled subscription or a crashed subscription will have
   zero rows in this view.  If the initial data synchronization of any
   table is in progress, there will be additional workers for the tables
   being synchronized. Moreover, if the
   streaming
   transaction is applied in parallel, there may be additional parallel apply
   workers.