How do I get the Metal device for IOS within a native plugin?

I'm making an existing (pre 5.2) plugin compatible with the later unity plugin format used by 5.2.3p1.
I've used the example code from
http://docs.unity3d.com/uploads/Examples/RenderingPluginExample53.zip
I have it working for Android and PC but in the sample plugin there is only D3D9, D3D11,D3D12 devices queryable.
I've seen reference to a file "Unity/IUnityGraphicsMetal.h" within the files here :
"https://bitbucket.org/Unity-Technologies/iosnativecodesamples" but I...

How do I get the Metal device for IOS within a native plugin?