Updated after Gen 9 release
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 almost always fail (eg Dark Void).
I used a Metronome team to beat Alola. I quite enjoy it but be prepared to repeat some battles many, many times. Gen 8 expanded the roster of Metronome Pokemon, and now Gen 9 has really opened the floodgates. I'll try to give a chronological guide to the different Pokemon, focusing on just the final evolutions.
Gen 1: Clefable (naturally), Mew (naturally), Primeape, Poliwrath, Alakazam, Machamp, Golem, Gengar, Hypno, Hitmonlee, Hitmonchan, Chansey, Mr. Mime (K), Jynx, Electabuzz, Magmar, Snorlax, Mewtwo
Gen 2 additions: Togetic, Granbull (breed), Smeargle (sketch)
Gen 3 additions: Politoed, Ursaring, Miltank, Blissey, Celebi, Ludicolo, Hariyama, Sableye, Medicham, Plusle, Minun, Volbeat, Illumise, Spinda, Kecleon, Banette, Dusclops, Jirachi
Gen 4 additions: Togekiss, Dusknoir
Gen 7 additions: Electivire, Magmortar
Gen 8 additions: Uxie, Mesprit, Azelf, Aromatisse, Slurpuff, Diancie, Inteleon, Polteageist, Grimmsnarl, Mr. Rime, Alcremie, Indeedee, Calyrex
Gen 9 additions: Wigglytuff, Golduck, Slowbro, Muk, Dragonite, Azumarill, Slowking, Gardevoir, Slaking, Swalot, Grumpig, Floatzel, Lucario, Weavile, Gothitelle, Florges, Toxtricity, Hatterene, Perserker, Grafaiai, Arboliva, Pawmot, Tinkaton, Annihilape, Scream Tail, Iron Hands, Iron Valiant
Sources: https://bulbapedia.bulbagarden.net/wiki/Metronome_(move)
https://game8.co/games/Pokemon-Scarlet-Violet/archives/389668
