The exporter's wal collector (enabled by default) calls pg_ls_waldir(),
which Postgres restricts to superusers and members of the pg_monitor
role. The exporter user is created as a plain managed-database user, so
scraping logged "permission denied for function pg_ls_waldir" on every
run.

Grant pg_monitor via the managed database's additional_sql_queries,
which the Postgres role applies both on initial creation and on
subsequent runs, covering existing installations too.

Fixes #3039

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>