A 70-year-old hedge fund manager from Allston pleaded guilty in federal court Wednesday to an array of counts in connection with a multimillion-dollar Ponzi scheme, prosecutors said. Raymond K. Montoya pleaded guilty to three counts of wire fraud, five counts of mail fraud, and two counts of conducting an unlawful monetary transaction, according to a statement from the US attorney’s office for Massachusetts. For an eight-year period ending in June 2017, Montoya ran a pooled investment hedge fund in Boston, authorities said. He told investors the fund was earning substantial returns.