Some of the hope Collin Cowgill provided the Mets is starting to drain away. The center fielder went 0-for-4 with two strikeouts in an 8-3 loss to the Phillies last night at Citizens Bank Park, further deepening Cowgill’s slump. He’s now in a 1-for-18 rut and is just 3-for-23 on the season, good for a .130 average. Or bad for a .130 average. That is the lowest average of any Met who has at least 20 at-bats this season. Granted, it’s a very small sample size. But Cowgill also doesn’t have the track record of an established player. “It’s definitely frustrating when you’re not having success,” he said last night. “I’m a competitor. I want to get a hit every time I’m in the box.” After being acquired from the A’s in the offseason, Cowgill emerged in spring training, hitting .303 (20-for-66) and earning the nod as the Opening Day center fielder. Then in the opener against the Padres, he drove in a career-high four runs on a grand slam — the first player ever to belt a grand slam in his Mets debut. He seemed like a promising addition, especially since the Mets have all kinds of outfield woes. The 26-year-old Cowgill hit a pinch-hit homer against the Marlins on Saturday for the lone hit in his last 18 at-bats. He also has not drawn a walk this season. “He played so well for us in spring training just by getting on base. Having some patience, getting good balls to hit,” Terry Collins said. “He’s got to get back into that mode.”