monitoring bdr nodes

I need some clarification on how to monitor BDR nodes. In particular determining replication lag. As an example, I have a two node cluster with nodes ‘A’ and ‘B’. I need to be able to look at node ‘B’ and determine if it is lagging behind node ‘A’, by interrogating node ‘B’ only.

From the BDR documentation on monitoring:

SELECT pg_xlog_location_diff(pg_current_xlog_insert_location(), flush_location) AS lag_bytes,
pid,
application_name
FROM pg_stat_replication;

Because it is querying the pg_stat_replication table, I will need to run this query on node ‘A’ to check the lag on node ‘B’, is that true? I need to be able run a query on node ‘B’ to determine if it node ‘B’ is behind. I am not sure the above query will work for that use case.