Hall of Fame second baseman Ryne Sandberg announced Monday that he’s battling prostate cancer. In a post on his Instagram page, the former Chicago Cubs great shared that he learned last week of his metastatic prostate cancer diagnosis, adding that he already has begun treatment. “We will continue to be positive, strong and fight to beat this,” Sandberg wrote in his post. “Please keep us in your thoughts and prayers during this difficult time for me and my family.” A bronze statue of Sandberg, 64, will be unveiled in Gallagher Way outside Wrigley Field on June 23, coinciding with the 40th anniv…