[Observability Onboarding] Add EBT event for Synthetics app getting rendered with data

[flash1293]

See also https://github.com/elastic/observability-dev/issues/3188#issuecomment-2027382446

To be able to track progress through onboarding flows, the initial render of the synthetics app after data is available (a "contentful render"), should result in an EBT event being emitted.

This allows to track whether data onboarding was successful in the sense of the user actually seeing their data in a curated form.

[elasticmachine]

Pinging @elastic/obs-ux-infra_services-team (Team:obs-ux-infra_services)

[akhileshpok]

@flash1293 - Agree, that this will be a useful enhancement. Thanks

[elasticmachine]

Pinging @elastic/obs-ux-onboarding-team (Feature: Observability Onboarding)

[elasticmachine]

Pinging @elastic/obs-ux-management-team (Team:obs-ux-management)

[shahzad31]

@flash1293 i belive this is already done #191703

[flash1293]

@shahzad31 that would be great! To be sure, does this mean it's only getting to this event if synthetics is set up already? If yes we can close this