In other words, what do you call these?
(Each individual side?)
Most generically, they would be called lapels (the left lapel and the right lapel). And, to the best of my knowledge, it's absolutely irrelevant whether it's a coat or a bathrobe that you're talking about—they're still called lapels. There are different types of lapels such as notch lapels, peak lapels and so on and so forth, but they're all lapels.