Can I have Clefairy's "Metronome" copy a GX attack, and if so does it use up my "one GX attack per game"?
Yes you can, and yes it does. Also if you've already used your GX attack for the game, then you cannot choose a GX attack with Metronome.
Source: TPCi Rules Team (2017-01-05), Sun & Moon FAQ (2017-02-02)

Relevant cards

Note: All applicable cards may not be listed below.
XY12_63