The 138th final of the FA Cup, the world’s oldest club competition, pits all-star Premier League champions Manchester City against able underdogs Watford. The odds may be heavily stacked in favour of Pep Guardiola’s team, but there is enough cause to believe that Javi Gracia can mastermind a memorable upset that would be a fitting conclusion to what has been a thrilling tournament this season. Here’s everything you need to know. WHERE: Wembley Stadium, London (capacity 90,000)WHEN: Saturday; noon ET, 5 p.m. UK (live on ESPN+) BACKSTORY: This is Watford’s second FA Cup final (35 years ago, they lost 2-0 to Everton) and for most of their history they’ve been a second-tier club, though since their most recent promotion to the Premier League in 2015 they’ve generally been solidly mid-table, finishing 11th this season. Manchester City are arguably the best team in the world right now and, in English terms, have completed the most dominant two seasons ever, having gained 198 of a possible 228 in winning back-to-back titles. They retained the Carabao Cup this year and would complete an unprecedented domestic treble if they win this too. MUSICAL ICON: Oasis’ Noel Gallagher is a fixture at City games, and…