The metal in a hip joint DOES NOT leak! You need to discuss your concerns with the Consultant who is going to perform the operation, I would make another appt. by ringing his/her Secretary (we don't bite ;-)), explaining my worries and asking for either a face-to-face consult or for him/her to ring me at home.
Have a list of queries ready so that you can discuss them with the Consultant. In fact, when I was working in the Hospital, the ball joint was plastic so there was a barrier between the joint into the upper thigh bone and the replacement in the hip joint itself.
How do you know that your "metal levels in my body are really high" - who has told you this and how has that conclusion been reached? Which metals in particular, remembering that the body is made up of various minerals etc.. Leaching of metals is rare, even the ones from teeth fillings!
However: if you are in any doubt about surgical intervention, you won't be in the 'right' frame of mind to make a good recovery. The important issue here is to talk over your concerns as soon as possible so that you don't 'waste' a theatre spot - no good waiting until you get into Hospital for surgery only to say 'no' because you are hesitant!
Let us know how you get on!