Deprecate telemetry.enabled: false

[afharo]

We currently have 2 ways of disabling telemetry:

1. telemetry.optIn: false and its similar Advanced Setting: we stop sending any data to our telemetry cluster
2. telemetry.enabled: false: the plugin is completely disabled in Kibana. Meaning its code is not loaded.

Following #89584, we want to reduce the ability to disable plugins to make testing easier. Additionally, the new #121992 client would benefit from the telemetry plugin not being disabled to flush its internal queues as soon as it's notified about opted-out.

What's the best way to transition into deprecating this value?

I tested that adding the following deprecation step in the telemetry config definition file works as expected:

export const config: PluginConfigDescriptor<TelemetryConfigType> = {
  schema: configSchema,
  exposeToBrowser: {
    enabled: true,
    banner: true,
    allowChangingOptInStatus: true,
    optIn: true,
    sendUsageFrom: true,
    sendUsageTo: true,
  },
  deprecations: ({ rename }) => [
    rename('enabled', 'optIn', {
      level: 'warning',
      message:
        'The Telemetry plugin cannot be disabled. To opt-out from usage reporting, set `telemetry.optIn` to `false`',
      correctiveActions: {
        manualSteps: [
          'Set `telemetry.optIn` to `false` in `kibana.yml`',
          'Remove `telemetry.enabled` in `kibana.yml`',
        ],
      },
    }),
  ],
};

However, there are some additional changes we may want to take into consideration:

• Should we allow allowChangingOptInStatus: false when optIn: false so Admins can fully disable any banners?
• We need to make sure that banners are not shown when they shouldn't. i.e.: If optIn: false and it cannot be changed via the UI allowChangingOptInStatus: false, it shouldn't show the Privacy Statement on top of not showing the opt-in/out UI options (Self-managed telemetry Privacy Statement notice shown in Cloud environment #110638).
• anything else??

[elasticmachine]

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

[Bamieh]

+1 for deprecating the enabled flag.

We prompt users to re-optin upon kibana upgrades. with enabled: false is set this behavior was completely disabled but now there will be no way to disable it directly, instead users need to set allowChangingOptInStatus: false in addition to optin: false.

[afharo]

We identified another potential win: https://discuss.elastic.co/t/kibana-wont-start-after-8-2-0-upgrade/304881

The user disabled telemetry via telemetry.enabled: false but the Telemetry SavedObject was previously created. The SO migrations failed because of the unknown telemetry SO type existed in the index, but was not registered by any plugin (again, the Telemetry plugin is now disabled).

Deprecating telemetry.enabled: false will solve this issue :)