PagerDuty, which offers an enterprise computing monitoring software as a service that has grown quite popular with companies looking to avoid downtime, became the latest tech company to join the public ranks Thursday. Shares in PagerDuty closed up 59 percent on the New York Stock Exchange after its first day of trading. The company graduated from the Y Combinator startup finishing school in 2010 and had raised $173.6 million from investors such as Accel, Andreessen Horowitz, and Bessemer Venture Partners, according to Crunchbase.