That one is the static port for the autopilot system. I take it your 172 has a KAP-140 autopilot? It uses that static port for the altitude hold which is why you also adjust the pressure setting on the autopilot. If you're not already you should start doing all three altimeters every time you get a new pressure setting. The altimeter, GPS and autopilot.

That little lip results in the Otto not working so well after a good rain. It traps rain water in the system.