Metagaming? Why wouldn't someone trained in a skill be able to estimate how hard a task in that particular area be for him or her? Why wouldn't a skilled combatant be able to judge the skill of an opponent after engaging or witnessing said opponent engage in combat? I see these as things that c.an be assesed and determined by the character in the fantasy world... I don't really consider it metagaming but to each their own.
But it is metagaming no matter how you consider it. Sure knowledges can reveal certain things like weaknesses, but they have never revealed things like AC, saves, and DC's.