Add device_id as an automatic context value for Feature Flags (#675)

* Initial plan

* Add device_id as automatic context value for Feature Flags

Co-authored-by: msiebert <1504059+msiebert@users.noreply.github.com>

* Add timeLastFetched and fetchLatencyMs tracking properties to $experiment_started events

Co-authored-by: msiebert <1504059+msiebert@users.noreply.github.com>

* Fix type mismatch in timeLastFetched and fetchLatencyMs properties

Co-authored-by: msiebert <1504059+msiebert@users.noreply.github.com>

* Fix private access level for timeLastFetched and fetchLatencyMs properties

Co-authored-by: msiebert <1504059+msiebert@users.noreply.github.com>

* fix concurrency bug

* determine if sync is needed when tracking

* add more tests

* tweak test

* prevent potential deadlock, simplify tracking threading

* add timing properties sanity test

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: msiebert <1504059+msiebert@users.noreply.github.com>
Co-authored-by: Mark Siebert <mark.siebert@mixpanel.com>
Co-authored-by: Jared McFarland <jared@mixpanel.com>

Author: 3 people