Metal is universally supported among Apple's product lines.
The point of OpenGL was to have an API universally supported everywhere
Both are os-specific graphics apis. It seems pretty comparable to me.
Also, see pjmlp's comments. Everything he said is spot on.
That was always more like a dream than reality. OpenGL implementations between GPU vendors and platforms all have significant "implementation defined behaviour", interesting bugs and different performance "hot paths". And as soon as you need to start using extensions, portability is out the window anyway. Even setting up a GL context across different operating systems (e.g. the very first thing every GL application needs to do) is a royal PITA unless bringing in additional helper libraries like GLFW or SDL.
Arguably the only actually portable GL flavour is WebGL.