Why does it seem like only the Germans had fighters equipped with ammo counters? I haven't done any kind of in-depth research yet but I wanted to ask this question here first. I'm aware that many Allied fighters did use colored tracers to indicate when ammo was low but it seems to me that was a bad idea since it could indicate to the enemy that you were low on ammo. An ammo counter gauge in the cockpit would seem to me much more convenient and efficient.

Yup, only saw german planes with that. As far as tracers are concerned, it's true that they work both ways, but as I've read many times, it was a real plus to have tracers for inexperimented pilots, so they could see where they were shooting. And you may also consider that the normal belting on a .50 is 4 API rounds, then a tracer one, and so on

F6F Hellcat and F2A Buffalo both have ammo counters. Avia 534 has windows in the ammo bins visible from the cockpit. There may be more, that's just off the top of my head.

Ammo counter on the F6F???

In IL2 the F6F has a dial ammo counter--I would guess that it accurately represents the real cockpit.