Normandy's Heartbeat is now exposed as a Nimbus feature: legacyHeartbeat. It's
argument structure is almost identical to the Normandy version, except all
arguments are nested under a "survey" property to prevent pollution of
experiments by simultaneous rollouts. (e.g., if an experiment were to omit an
optional parameter but a simultaneous rollout included that parameter, Nimbus
would return the value from the rollout when showing the Hearbeat for the
experiment.)
Differential Revision: https://phabricator.services.mozilla.com/D169409
