Slight draft felt from window jamb
Apartment buildings are notorious for poor weatherproofing - especially around windows. If there is a gap that cold air is passing through you can seal it with a can of spray insulating foam. Better yet have the apartment super do it for you.
But the heat loss is probably the result of poor weatherizing. Those metal windows are heat bridges conducting heat out of your apartment. In an apartment you have no option to upgrade windows so the best answer is warm window covers (blinds or curtains) that can seal around the window and reduce your heat loss. They can be costly but at least you'll be comfy and reduce your energy costs. However, if this is just a short term rental, anything that has reasonable insulating properties that you can hang or tape over the window should help. Otherwise, wear a heavy sweater.