If your goal is to learn Metal, then just start with Metal. It's a significantly better-designed API. On top of that, if you're developing on an Apple device then you'll have a decent debugging experience with Metal, as opposed to OpenGL where you'll get... nothing last I checked (but it's been a while).
Is not OpenGL a good starting point if you want to transition to Metal or Vulkan later on?
If not, should I seek out the "new flavor" to learn graphics?