Query on pg_stat_activity table got stuck

Hello,

We are using PG 10.6. We have one cron job that queries pg_stat_activity
table to find out how many queries are running longer than X minutes and
generate metrics.

Query look like this :
SELECT * FROM pg_stat_activity WHERE state='active'

After some days, this query get stuck and doesn't finish. We tried to run
this query manually and same result.

We looked into pg_locks table and there this query is not blocked on any
lock :
https://justpaste.it/48rpe

Now we don't know where to look next. Our expectation was query on
pg_stat_activity shouldn't get blocked on anything and pg_locks also
showing same result.
Google search also not returning anything useful that is why reaching out
to this email list.
We have separated that DB and issue is still produceable on that DB, in
case if more information is not needed.

-------------------------------------
Thanks
Neeraj