Metrognome is a rare Artifact card from Urza's Saga (USG), collector number 301. Current market price: $0.47, TCGplayer market prices via Scryfall.
When a spell or ability an opponent controls causes you to discard this card, create four 1/1 colorless Gnome artifact creature tokens. {4}, {T}: Create a 1/1 colorless Gnome artifact creature token.