Oktoberfest is open. Mayor Dieter Reiter officially started Oktoberfest at noon on Saturday when he inserted the tap into the first beer keg, signalling the 189th start of the festival. Thousands of beer lovers celebrated in the Munich fairground as the first to clink their steins during the world’s largest folk festival. Servers immediately began ferrying trays — each carrying up to eight glass mugs — to tables. The celebration runs through October 6 in 18 large tents covering the Theresienwiese fairground. This year’s festival includes stepped-up security in the wake of a deadly knife attack…