The ele key is assumed to be measured in meters (see https://wiki.openstreetmap.org/wiki/Key:ele). Feet is technically not yet supposed to be used but should be indicated at least using a typewriter apostrophe character after the feet value (see https://community.openstreetmap.org/t/rfc-documenting-feet-as-an-an-optional-elevation-unit/108543 for more info).