Delightful question. Given the root of Metris that Mercedes talks about, that is, "Mercedes chose for our version the name 'Metris,' in reference to the metropolitan areas it figures most Metris owners will ply their trades in" (http://www.caranddriver.com/reviews/2016-mercedes-benz-metris-passenger-van-test-review), the most similar word in English would be, it seems to me, metropolis. The plural of metropolis is metropolises. But since metropolis comes from Latin (which took it from Greek), those of us who are old fashioned (e.g., 1800 years behind the times), might prefer the Latin plural of metropolis which is, in what seems to be the most relevant context, metropolium (https://en.wiktionary.org/wiki/metropolium#Latin). So I think either Metrises or Metrium would work.