Get transactional email metrics

Unified engagement metrics returned by analytics endpoints. When a period or start/end filter is applied these are a funnel over the sends made inside that window, not a log of events inside it: every delivery, engagement, and reply count is attributed to one of those sends, including activity that arrives after the window closes, so opened <= delivered <= sent always holds and no rate can exceed 100%. conversions and revenueCents are the exception and use the goal's own last-touch attribution window. Open and click metrics exclude detected scanner, preview, and tracked asset events unless includeMachineEngagement is true.