[Telemetry] Deprecate TelemetryStart.getIsOptedIn() in favour of an observer

[afharo]

#122146 identified an issue with getIsOptedIn.
The solution is OK for now, but it does not make sense to fetch the SO every time the API is called and expose potential ES connection glitches to the plugins calling this getter.

Ideally, this API should be an observer that emits whenever the value changes.

This issue is to create the new observer-based API and to deprecate the current API.

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[pgayvallet]

and to deprecate the current API.

How many usages of the getIsOptedIn API do we currently have? Would it be worth it to adapt the existing calls directly instead of deprecating the old API?

[afharo]

AFAIK, it's used in just a few places:

• In the UI
  1. The Welcome screen in the Home plugin - to show the opt-in/out banner
  2. The License Management - to show the opt-in/out banner
  3. The Telemetry Management Section - to handle the opt-in/out setting
• In the Server:
  1. Security Solutions' EBT sender
  2. Fleet's EBT sender

++ to replacing them straight away. Especially on the server-side which is the main SO fetcher trigger :)

[afharo]

Increased the priority because the number of clients is growing

[afharo]

After the EBT project, the opt-in status is now held by an observer. Applying these changes should be fairly simple. Adding the good first issue label.

[TinaHeiligers]

@afharo this issue has had an update in 2 years. I'm closing as "stale" and we can open it again if needed.