That's not a pore, it's just a tear in the veneer which is a lot deeper. I've seen people say that it takes a stain, but you might be best waiting until the stain arrives and you can try a test piece on some scrap. The veneer won't need grain filling apart from in that slight tear, so only fill that small area. I's try 180 or 240 grit, nothing smaller, and nothing to coarse either. You just want to try and rough up the surface without removing any depth, so drag it across the top applying only minimal downward pressure.