Why does skin stick to frozen metal?

If you put your tongue on any frozen metal, the high conductivity of the metal removes the heat from your tongue much faster than your body can supply it. This makes the saliva freeze into ice very quickly. This ice creates hydrogen bonds between the metal and your skin, just like water does, so the whole surface bonds together and resists as you pull away. This same thing will happen with your fngers because your fingerprints provide the roughness needed and your fingers always have some moisture due to sweat. However very smooth, dry skin won't stick. 

Why does a mint make ice cream even colder?

All mints contain the chemical menthol, which binds to the TRP-M8 receptors in your skin which sense cold, the temperature doesn't change it's the menthol that causes false cold signals to be sent to your brain. This feeling is intensified because the ice cream also sends cold signals to your brain and this increases the number of cold receptors that are firing at once. This is similar to the reaction caused by chilli which is caused by the chemical capsaicin which binds to the TRP-V1 receptors for heat.