She uses a meter:
As in a measuring device which could include a speedometer, a sound meter, odometer, voltmeter, ohmmeter, frequency meter, water meter, gas meter electrical meter, radon meter, oscilloscope, Geiger counter...
Some of these are meters which equal metros as in Velocímetro = speedometer; ohmímetro = ohmmeter; voltímetro = voltmeter but a water meter = medidor de água (this is also the word for meter used in poems as in medidor de palavras but, Pentâmetro iâmbico = iambic pentameter).
She uses one meter:
She uses a substance such as cloth, string, wood, or other similarly measured item in the specific length of one meter.
She uses one/a metro:
Metro as in the transport system does not have the accent in EU PT (Europe, Asia, Africa) as it does in Brazil.
Subway is basically unique to NYC (maybe East Coast, North America) as most places I've been have Metros or an Underground instead now (US included) and the Tube in London (and not all are under, or under all the time). A subway in the UK is a pedestrian underpass, usually underground that bypasses a street (do not have to wait to cross it). The one at Elephant & Castle is majorly confusing. Can't be sure where you'll pop up till you've done it a few dozen times. =\
This is indeed a confusing sentence. =]