Dominic Smith learned a hard lesson Friday under new Mets manager Mickey Callaway. Show up late, take a seat. Smith, who knew Thursday afternoon he was penciled in to start Friday’s Grapefruit League opener, didn’t get to the clubhouse in time for the 8:45 a.m. pregame meeting. As a result, Callaway took down the original lineup card, replaced Smith with Peter Alonso — the first base prospect from Double-A Binghamton — and then Smith had to explain himself. Initially, Smith didn’t reveal the reason for his benching when asked by reporters. But a few minutes later, Smith was called into Callaway’s office, and after a five-minute chat, returned to his locker, where he was then counseled by two Mets media relations personnel. Moments later, reporters were waved back over and Smith copped to the offense. “Not that late, but late enough for it to be a problem for sure,” Smith said. “I shouldn’t be cutting it close like that. I’m a professional. This is my job. This is my career. It’s my livelihood. I felt like I definitely did let them down today.” Smith already is ticketed for Triple-A Las Vegas, after the Mets signed veteran Adrian Gonzalez this winter, and he certainly didn’t help his cause by blowing his first opportunity to start, even for a spring training game. Smith had earned plenty of positive attention for his weight loss during the offseason, but this is not the way to kick off what already is an uphill battle to get to the big leagues.