Cleaning a metal coffee table is an easy task you can accomplish in a relatively short period of time using supplies you most likely have around your home.
Step 1 - Determine Metal Type
Before you clean, you need to determine what type of metal you are cleaning. Most likely the top is steel or galvanized or tin. All of these would be cleaned the same way. With a soft cloth, dust the entire table to get anything off it that would scratch the metal. Then, take a damp cloth and wipe away any dirt or stains. You can also use a household general cleaner if the top is especially dirty or has sticky or messy spills on it.
If the top is dull, shine with a stainless steel cleaner or use toothpaste on a soft cloth and buff the area.
Step 2 - Cleaning a Copper Table Top
A copper table top may or may not have a lacquer coating over the copper. If it has a lacquer coating, just wipe clean with a damp cloth. If you see some dark discoloration or green on the copper, it doesn't have a lacquer coating. You can use half a lemon dipped in salt to scour it or any good commercial copper cleaner such as Barkeepers Friend.