Too much data queued in replication slots
You are measuring how much WAL you have retained on the primary not yet sent to the replica.  But the hard drive filling up is the one on the replica, not the one on the master, so this is not the right thing to measure.  

So what you need to measure is the amount of WAL received by the replica, but not yet replayed and eligible for removal.  You can do this by connecting to the replica (if it is in hot-standby mode) and running pg_last_wal_receive_lsn() and  pg_last_wal_replay_lsn().  But that is probably not quite right either, as just because it has been replayed doesn't mean it can be and has been removed, it still has to wait for the restartpoint.  So it would probably be better to check the disk space directly, for example by using ssh to run a df command.