The Eleventh Metal is a short story originally written for the Mistborn Adventure Game (an RPG) that takes place before the events of the first Mistborn book. It gives us a bit more backstory about Kelsier and actually shows his own training, which is interesting enough.
Overall, it’s not bad. It’s just very short and doesn’t really add anything in particular to the rest of the Mistborn stories.
Worth the read mainly because it’s short, but also easy enough to skip.