All minerals: metal, crystal, stone, rock, sand, glass, etcetera, are aspects of the earth element. The earth Druid attunes to metal.Not really. The earth is composed of many things, not just metal. In this case too much metal conflicts with the earth.
In a narrative, where animals and plants protect the Druid:If it were written like that, they would indeed not be proficient with metal armor, which is nonsensical as one kind of armor functions the same as the other.
All of the abilities that give partial proficiency are very explicit. Elves get proficiency with all longswords(regardless of material). Dwarves get proficiency with all battleaxes, light hammers and warhammers. Pact warlocks don't get proficiency, but use their pact weapon with proficiency.
Not one instance of partial proficiency fails to spell it out, which is why druids don't have partial proficiency. It does not spell it out.
The Druid can wear animal-plant armors proficiently, almost exactly the same way the Warlock can wield a pact weapon proficiently.