The only free and comprehensive online etymological dictionary of the Spanish language
metro (1) m. (Noun) "meter"
Late 18th cent. Either borrowed from French mètre 'id.' and then from Ancient Greek μέτρον ‎(métron) "measure" or borrowed directly from Ancient Greek, bypassing French. From Proto-Indo-European *mh1-tro- "measurement." From the root *meh1- "to measure" (see also medir).
Indo-European
Romance
French mètre, Italian metro
Celtic
Old Irish midithir "to measure," Middle Welsh meðu "to think," meddu "rule," Middle Cornish medhes "to say"
Germanic
Gothic mitan "to measure," Old Norse meta "to evaluate," Old High German mezzan "to measure," Old English metan 'id.' (English to measure)
Hellenic
Ancient Greek μέδομαι (médomai) "to care for"
Armenian
Armenian mit-kʿ "mind"
Indo-Iranian
Young Avestan vī-māδaiiaṇta "they must measure"