MetTel, a New York City telecom services company, has agreed to buy Thrive Networks, a Tewksbury unit of the office supply giant Staples Inc. that provides information technology services to businesses. MetTel did not disclose financial details about the sales agreement. Thrive’s 71 engineers will remain in Tewksbury, MetTel said. MetTel, with 254 employees, projects 2014 revenue of about $300 million. Thrive was founded in 2000 and acquired by Staples in 2006. Staples is reinventing itself as it faces intensifying competition.