We use the following simple script/Docker container at Magnet.me monitor the lag or our postgresql slaves continuously.
You can use the container like this:
docker run \
-e INFLUX_URL=http://root:root@localhost:8096/infra \
-e RATE=250 \
-e PGUSER=replication \
-e PGPASSWORD=yoursecret \
-e SERVERS=localhost:5433,localhost:5434 \
magnetme/postgresql-replication-lag-influx