Depends on the type of fire extinguisher really. Most extinguishers work to suffocate the fire (e.g. ABC or CO2 extinguishers), but depending on the type of burning metal the chemical(s) in the extinguisher might react with the metal and make the fire worse. They make special "class D" extinguishers specifically for metal fires, which spray a non-reactive powder (usually graphite dust I think) over the metal to suffocate it.
At best nothing, at worst you spread the fire around by moving the burning metal.