If the Druid takes heavy armor proficiency via a feat, this same feat would grant proficiency with metal armors. Thus all armors become available.
Technically, they could only wear heavy armor that was metal. But I would handwaive that to mean, becoming proficient with metal generally, thus can wear chain, scale, and studded leather too.
(Of course, both studded leather and ring armor are erroneous armors, mistaking stylized illustrations of brigandine and chain, respectively. However, ring armor can be real in the sense of a very inferior scale armor, where metal is scarce.)