Metronome converts to a randomly chosen move, the power and accuracy of the move do not change. As Staka says, the Normalium Z will convert a random move into the corresponding Z-move, for example, Metronome selecting Razor Leaf used as a Z-move will yield Bloom Doom. It can't choose every single move (see Bulbapedia for exceptions) and some moves it can select will always fail (eg Dark Void).
I recently used a Metronome team to beat Alola. I quite enjoy it but be prepared to repeat some battles many, many times. Currently there are around 35 Pokémon in their final evolutions that can legitimately learn the move.
Natural (level up/breeding): Mew, Clefable, Blissey, Snorlax, Granbull, Togekiss
Transfer (Gen 1/Gen 3): Alakazam, Hypno, Golem, Politoed, Poliwrath, Primeape, Hitmonlee, Hitmonchan, Jynx, Mr. Mime, Machamp, Mewtwo, Ludicolo, Hariyama, Sableye, Spinda, Kecleon, Banette, Dusknoir, Ambipom, Ursaring, Plusle, Minun, Volbeat, Illumise, Miltank, Medicham, Magmortar, Electivire, Gengar, Smeargle, Jirachi
learn move in pre-evolution