Is every subset of a metric space a metric subspace? A simple proof does justify that all are subspaces, still, wanted to know if I missed something.
To be completely precise, all subsets of a metric space equipped with the induced metric are metric subspaces. You could also equip the subsets with other metrics, and then they wouldn't be metric subspaces.
(Strictly speaking you could also not equip them with a metric at all, and then they aren't metric subspaces because they aren't even metric spaces).