Is asking someone's lvl metagaming?

If yes, Then how do you know you aren't getting a dm mad if you decide to pummel someoen in the sewers?

If you're following all the rules of PVP, then level doesn't really matter.

Asking someone's level isn't metagaming, but it's annoying. It becomes metagaming if you use that knowledge to your advantage in an IC fashion, which presumably is what one is attempting if they go out of their way to ask.

And yes, as Metro states, if you follow all the PvP rules (which you should know in full before you initiate any PvP) then levels do not matter.