[Monitoring] Move stats collector back into the monitoring plugin

[chrisronline]

#62665 introduced a weird relationship between Kibana monitoring and telemetry when we moved the kibana_stats collector outside of monitoring and into the telemetry plugin. This collector is responsible to collect Kibana monitoring metrics, which we then ship to the monitoring cluster. Unfortunately, by moving the collector into the telemetry plugin, it means that one could disable the telemetry plugin and it essentially breaks Kibana monitoring.

#65663 unintentionally fixed this issue by moving it to another plugin, but we still have the same issues.

I understand that we wanted to move the stats collector into a pure OSS environment, but it presents an awkward situation where a seemingly random plugin is disabled and Kibana monitoring breaks.

I think we need to move this back into the monitoring plugin and take another pass at how to handle the OSS issue.

[elasticmachine]

Pinging @elastic/stack-monitoring (Team:Monitoring)

[afharo]

I think we can close this issue. #83546 already removed that dependency :)

@chrisonline, feel free to reopen if you feel otherwise.