Metronome is able to call Z-Moves and Max Moves.
I had been spamming Metronome against several wild Pokemon for 1 hour. During this time, Metronome called Max Wyrmwind, Clangorous Soulblaze, G-Max Hydrosnipe, Max Hailstorm, G-Max Meltdown, G-Max Terror, 10,000,000 Volt Thunderbolt, G-Max Rapid Flow, G-Max Finale, G-Max Cuddle, Max Airstream, Max Geyser, G-Max Snooze, Max Ooze, Max Darkness, Let's Snuggle Together, and Guardian of Alola.
Based on these results, it would appear that Metronome can call any Max move or G-Max move, as well as Z-Moves that are specific to one Pokemon. Metronome is not supposed to be able to call these moves.
Tested using The Pixelmon Modpack on CurseForge.