WATERTOWN — Northern Federal Credit Union will soon have its name on the Watertown Municipal Arena. The City Council is expected to approve an agreement on Monday night for the credit union to spend $100,000 for the naming rights of the arena. On June 2, the credit union was the high bidder when sealed bids were opened. In April, the purchasing department issued a request for proposals, or RFP, looking for someone to rename the city’s ice rink at the Alex T. Duffy Fairgrounds.