The first installment of this year’s Subway Series features two teams that have spent most of the season heading in opposite directions, with the Yankees among the top teams in the American League and the Mets among the worst in baseball — and certainly the most disappointing. But season results haven’t always mattered when the crosstown rivals meet, and the Yankees are in the middle of a rough road trip and perhaps their worst stretch of the season, while the Mets completed their first sweep of the year Thursday. The Citi Field crowd will be looking for any reason to cheer as the Mets play host to this series beginning Friday at Citi Field. At the plate Getting on base It’s hard to overstate just how bad the Mets offense has been, but we can start here: They entered Thursday with the lowest on-base percentage in the majors and it’s easy to see why.…