When leaders of the 109-year-old Isabella Stewart Gardner Museum unveiled its $114 million addition today, they did so with an eye toward the past, referencing the project not as an expansion but as a “preservation.’’ Isabella Stewart Gardner herself could never have anticipated that her palace-museum would grow to the point that roughly 200,000 visitors stream in each year, officials said. The influx created stress on the palace and made it hard for the Gardner Museum to expand its programs. The new building should solve those problems.