Vaious D&D 5e sourcebooks mention "metal armor" or "armor made of metal". Here's a few examples:
PHB page 65:
druids will not wear armor or use shields made of metal
PHB page 275:
You have advantage on the attack roll if the target is wearing armor made of metal
Monster Manual, page 262:
If the object touched is either metal armor or a metal shield being worn or carried, its takes a permanent and cumulative -1 penalty to the AC it offers
However, almost any armor has metal elements in it, but few armor is completely made of metal with no other materials. So what is "metal armor"? And what is "armor made of metal"?
For instance, the Studded leather armor is described as
Made from tough but flexible leather, studded leather is reinforced with close-set rivets or spikes.
Providing rivets or spikes are made of metal, does it make the whole armor "made of metal"? Can a druid wear it? Does a Rust Monster do damage to it?
Moreover, even a simple leather armor migh have metal closings/fibulas:
On the other hand, a full-plate gauntlet has non-metal elements (cloth or leather):