By SUSAN SCHROCK ARLINGTON — Dallas Cowboys fans may have a reason to visit Arlington City Hall soon — cheaper stadium parking.The City Council is set to vote Tuesday whether to open up 746 city-owned parking spaces downtown, including those at City Hall, for events in the entertainment district. The lots would be privately operated by AAA Parking, and Arlington would collect $7 for each space sold, generating an estimated $130,550 annually."I really don’t see any downside to converting some of the city property to parking during events," north Arlington council member Mel LeBlanc said.