The Brief: The phrase "metal" or "that's metal" means that something is hardcore, much like metal music.

DEEP DIVE
Variations of “that’s metal” include “that’s so metal,” “now that’s metal,” “that’s metal AF,” and simply “metal.” Synonyms for metal include bada**, savage, or wicked.
“Metal” can be used in direct reference to metal music, or metaphorically for something that resembles the musical genre in its hardcore-ness.
a my chemical romance reunion announcement on halloween? thatβs metal.
— Morgan Guidry (@mo_arianna) October 31, 2019
That's metal af (no pun intended) https://t.co/LZILt8smXN
— πππππππππ.βΎ (@pendragontwt) October 30, 2019
βWe stand on guard for thee?β Thatβs metal af and makes me want to charge into battle. And Iβm not even Canadian!
— Clay Winstead (@ClayWinstead) October 31, 2019