Keep in mind that you can trigger items you haven't unlocked yet, which means you could trigger the effect of items like Death Certificate without having to go through the trouble of unlocking them.
If Metronome triggers the effect of Genesis, it will disappear from the active slot, just as with the real item.
Blanket / Holy Card / Holy Mantle / Wooden Cross: if Metronome is used while the shield is active, any stat changes or familiars/orbitals generated from Metronome will persist between rooms until Metronome is used again or replaced with another active item. Items that modify your stats and have other additional effects will keep the stats but the effect will be gone after leaving the room. For example, the tears down applied from The Inner Eye will persist between rooms, but Isaac will not shoot three tears at once.
Some tear effects from items will remain active between rooms. These include:
PC It is possible for this item to use an item that can teleport one to the I AM ERROR room. During the Mother Boss fight, this can happen. Using the trapdoor causes the game to crash.
PC It is possible for this item to give an indefinite Breath of Life effect that persists throughout rooms, eventually killing the player.
PC If Urn of Souls is activated through Metronome, it won't shoot fire and Isaac could get stuck with the urn in his hands unable to shoot tears. You can fix this by exiting and continuing the run, restarting the room without the urn and without Metronome charges.
PC Using Metronome in the Cantripped! challenge can cause the game to crash.