Update pg_stat grant statement to not throw error
While this doesn't fix query stats collection, it is necessary all the same.
Before this change: https://ops.gitlab.net/gitlab-com/database-team/gitlab-com-database-testing/-/jobs/11325248#L46
With this change: https://ops.gitlab.net/gitlab-com/database-team/gitlab-com-database-testing/-/jobs/11325567#L70