Last week in our game, my players faced some Drow in a small dungeon encounter. One of my players, playing a 15 year old tiefling Sorceror (level 1) chose not to cast Sleep on the Drow because he (the player) knew it would be ineffective. I didn't give it much thought until a few days ago when I realized it was kind of a metagame decision. I was going to ask him about it, when I realized that all elves in 5e have the racial feature Fey Ancestry, and the tiefling character grew up in an elven city, so it might be a moot point. Even without his IC justification at the time it could still be justified IC, as long as it is common knowledge the Fey Ancestry grants immunity to sleep.
So, simple question: Is the elven racial feature Fey Ancestry commonly held knowledge, and would any person simply know that elves can't be put to sleep due to their connection with the fey?
Though we are playing in a custom setting, it is loosely analogous to the Forgotten Realms with regards to the setting and lore, so examples from published works in that setting that point to this being common knowledge would be helpful.