dashboard-update-add-service-variable

Instruction

We'd like a service dropdown on "Service Overview" (`service-overview`) so on-call can scope the whole board to one or more services - and still flip back to All when needed. Back it with the backend Prometheus `job` values that emit request metrics here, and make the saved metric queries keep working when someone picks one service, several services, or All from the saved dropdown. Can you add that without wrecking what's already there?