Here at SFist, we are not impressed with percussive instruments. Calling a drum circle "music" is like calling crosswalk stripes "a painting." But there's something sort of impressive about an experienced cable car bell-ringer, so gosh darn it we can't quite manage to fully roll our eyes at today's annual Muni Cable Car Bell Ringing Contest (noon in Union Square), at which Gavin Newsom and several local news personalities (please Jan Wahl, please Jan......