Major Description
Now that the slave exports per-executor stats via the UI, the slave should be able to periodically request application-specific stats from the executor (e.g. zookeeper session id, connection counts, whatever) and export them on the executor's behalf. This way we don't need to reinvent any observability wheels.
The interface could either be exportStats(task_id) or just exportStats(), and return a JSON object with all stats to export.