These words, although sounding the same, have different meanings.
Meddle is a verb meaning to interfere:
Mr. Troy’s evil plan was stopped by meddling do-gooders.
Mettle is a noun for a kind of strength—not the kind of strength needed for lifting heavy weights, but rather spiritual strength, courage, or general toughness—the ability to stand against threatening forces:
After being a soldier at war, nothing else could test his mettle.
To test someone’s mettle is a very popular cliché for any challenge:
Tasting this hot pepper will test your mettle!