Sometimes...

If a metal is being painted for the first time or will likely be prone to oxidisation, then a primer is highly recommended. It will combat corrosion and rust. Again, if it is in any sort of disrepute, flaking or rusted then a primer is essential when addressing the longevity of your project. 

There will be situations when a metal primer may not be essential. If your paint already on the metal is in good condition and you are simply adding another coat, the properties that prevent moisture seeping in would appear to be in play. Stainless steel has compounds that combat oxidation.

However! Primer is there for a reason. It bonds to the substrate better than topcoats, it will prevent rusting and flaking that may occur earlier without its presence. Not using primer is a chance to cut a corner that may end up costing you a lot more when the job does not last as long.

There are also some excellent direct to metal paints available now and when circumstances permit these can be an excellent option to save time and money.