The Americans spell it "Meter" but the preferred way of spelling it everywhere else (English wise) is "Metre". Usually Meter everywhere else (Outside the States) would indicate an actual machine or gadget designed to measure time, distance, speed, intensity or to regulate current. But since this site is based out of the states that's why the main spelling is "meter". If you do put Metre though it does accept it since it is a proper way of spelling it
It is never "Einz Meter" since there is no such word as "einz".
You are perhaps thinking "ein
s Meter", but one uses "eins" when counting, but not when describing how many there are of something. Then one uses "ein", with no "s:. This means there is the potential for confusion between ein meaning "one" verses meaning "a/an" (the indefinite article for masculine and neuter nouns). See https://en.wiktionary.org/wiki/ein#Usage_notes_4 for how that confusion can be addressed.