A druid gains medium armor proficiency, but is not allowed to use metal armor. A warforged with medium armor proficiency can normally set their Integrated Protection to Composite Plating, granting an AC of 13 + prof. bonus + Dex mod (max 2).
Does Composite Plating count as metal armor for the purposes of a druid's armor restrictions? Also, if a warforged druid later gains heavy armor proficiency, can they use the Heavy Plating mode? In other words, is a warforged druid restricted to only the Darkwood Core mode?
The warforged race is described in Wayfinder's Guide to Eberron. (The version this question asks about also appears in the Eberron Races UA; as Wayfinder's Guide to Eberron is a living document, it will likely be updated based on UA feedback at some point in the future.)