ATLANTA -- Kroger Co., Cincinnati, reportedly plans to open 12 new stores in the metropolitan area here in 1996, including an experimental ministore in the downtown area. Company officials declined to comment. Local observers said the ministore is scheduled to open in May across from City Hall. They said it is designed to draw more residents downtown, where their shopping choices currently include only small convenience stores with limited selections and hours. Kroger operates 139 stores in its Atlanta division (which encompasses Georgia and parts of South Carolina), including 76 stores in the metropolitan area. It opened seven new stores here in 1994 and is on target to add 10 this year. Beyond the 12 stores reportedly projected for next year, observers said Kroger plans 10 additional metro-area stores in 1997.