It’s been well over a year and a half since Clay Holmes last wore pinstripes.  As the 6-foot-5 hurler prepares to make his first start against the Yankees — now as a member of the rival Mets — in the Subway Series opener Friday, Holmes has allowed himself to reflect on how far he’s come since the thought of transitioning back into a starting role lingered in his mind while playing in The Bronx.  “I try to sit back sometimes and just like, ‘Man, this kind of went how I wanted it to,’ ” Holmes told The Post before the Mets’ 9-4 win over the Tigers on Thursday to cap a series sweep. “I think there’s a lot of gratitude with that of just being able to just take a chance and seeing it pay off. It’s hard to do that. I mean, you’re in the day to day and you’re in your preparation and it’s like you wake up every day with something you got to accomplish.  Clay Holmes prepares to throw a pitch during the Mets’ April 28 game.…