I will stop it just like your Nurse advised. Monitor your heart rate. How fast was it that they had you start Metoprolol ? If your heart beat gets rapid again before your appointment, call your MD again.
What you hear is true - Its not good to stop meds if you have been on it and it has been maintaining your heart rate. This is different. you just started and you are having a reaction. Your MD will probably give you another RX.(Prescription) in its place.
Take care
I can't give you advice on stopping a medication cold turkey but I can tell you that if you are not comfortable doing this after speaking to your doctor you should talk to your pharmacist. From what I know it is a low dose. If you haven't stopped it yet maybe consider stopping it on Monday when the doctors office is open if you have any troubles and need to be seen or need more advice. Hope you feel better.