[Ingest Manager] Improve agent unenrollment

[nchaulet]

Description

Related to elastic/kibana#67409

We need to improve the way we unenroll agent.
Currently it's done by revoking all API keys used by the agent, and the agent guess that it should unenroll if he start receiving 401.

We can to improve this to support:

• a gracefull unenrollment, that give time to the agent to send back all the information we want to both Kibana and ES
• the agent need to uninstall endpoint
• we still need to support a way to instantly unenroll agent where we revoke all API keys

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[EricDavisX]

I'm posting this here and in elastic/kibana#67704 -

• one concern I have that I don't see commentary on, is making sure we have and provide in the API call the time that the agent was unenrolled (from the UI).

from api/ingest_manager/fleet/agents?page=1&perPage=20&showInactive=true

• you can see the enrolled_at timestamp field, we should capture and add to the metadata an unenrolled_at timestamp. that field can be empty if we want / ned to populate it in the first place, and when empty it need not have any impact on the UI.

Also, do we ever intend to show users the list of inactive Agents in the UI? It may certainly be useful, not sure that I saw tickets for that for Beta 1 or beyond, but I assume we have to... @mostlyjason yes?

[EricDavisX]

Same concern for when we revoke an enrollment API key - I don't think that was tracked either, it may be an even bigger concern for that API + use case.

[mostlyjason]

Sorry I missed this one earlier @EricDavisX. yes I think we show the inactive ones in the UI. They are hidden by default but there is a filter to show them.

[ph]

@nchaulet Can you confirm the behavior in #18825 (comment) and #18825 (comment)