Answer:
I am a portal to another world which you cannot enter. Only you can see me but I can't see you.
I'm sometimes white but most often I'm black. I take you there but never bring you back.
My voice is tender my waist is slender and I'm often invited to play. Yet wherever I go I must take my bow or else I have nothing to say.
I am a king who's good at measuring stuff.
I can speak with my hard metal tongue. But I cannot breathe for I have no lung
Runs over fields and woods all day. Under the bed at night sits not alone, With long tongue hanging out, Resting at your feet until we go for a walk