Armour is a specifically defined mechanic. The list of what is considered armour can be found on PH pages 144–145.
Plate armour includes a helm, but none of the other armours list a helmet as part of the garb.
Because druids are only restricted in the armour they wear (per PH65), they can wear magical hats, helms1, circlets, tiaras, and any other piece of headwear that is made of metal.
Would a Druid ever want to wear a helmet if it is made of metal?
Yes, there are several magical wondrous items (not armour) that fit on the head that are made of metal.
Are helmets considered part of the armor?
Only with plate armour, where it's specifically called out that it includes a helmet. No other metal armour is noted to include a helm.
1Helms that are not part of plate armour. For example the helm of telepathy & helm of teleportation. They are all categorized as wondrous items, not armour.