In PostgreSQL Agent 2 template documentation:

https://git.zabbix.com/projects/ZBX/repos/zabbix/browse/templates/db/postgresql_agent2?at=release/5.0

In section

1. Create PostgreSQL user for monitoring (<password> at your discretion):

Missing important GRANT section:

GRANT EXECUTE ON FUNCTION pg_ls_waldir() TO zbx_monitor;

Otherwise in fronted error appear
Cannot fetch data: ERROR: permission denied for function pg_ls_waldir (SQLSTATE 42501).