Guess what? You can do that already thanks to a variety of third-party apps, many of which debuted last year when Apple first introduced its ARKit software (now with multiuser support).
For example, there's Laan Labs' Air Measure (formerly AR Measure), which works much like iOS Measure. You simply point your camera at a starting point, tap a button, then move it to the end point and tap again. A virtual tape-measure appears as you move.
Air Measure can also measure the vertical height of an object, generate a 3D cube around an object (to help you determine what size shipping box you might need, for example) and help you build an accurate 2D floorplan. It's a free app.
Bottom line: You don't need to wait for iOS 12 to add an augmented-reality tape measure to your iPhone. You do, however, need an iPhone that's ARKit-compatible, meaning an iPhone 6S or later.
Search the App Store for "AR measure" and you'll find lots of others, with prices ranging from free to about $10.