Telemetry opt-in setting

[legrego]

The Spaces plugin will be making the Advanced UI Settings space-aware. There are a couple of UI Settings that do not make sense to be space-specific, such as telemetry:optIn. The decision to enable telemetry should be made for the entire Kibana instance, rather than for each individual space within the instance.

Instead of supporting one-off cases for specific configuration settings like this one, we are proposing that we remove the telemetry:optIn setting from the config document in favor of a dedicated document in the .kibana index which tracks the telemetry opt-in preference.

Given this, we propose the following:

1. On Kibana server startup, migrate the user's existing telemetry:optIn preference (if set) to the dedicated 'telemetry' document
2. Expose a set of read/write API Endpoints for the new 'telemetry' document
3. Create a new screen in the Management section to configure the telemetry decision.
4. Update the opt-in banner and telemetry collector to use the new API Endpoints.

Since the monitoring xpack-main plugin writes the current telemetry settings, we propose making these changes within the monitoring xpack_main plugin, with the understanding that a dedicated telemetry plugin may be created in the future.

@kobelb @tsullivan @pickypg

[pickypg]

Technically, xpack_main writes these settings now.

[jbudz]

Could this be generalized to config and admin config? I expect there's a few others, like xpack:defaultAdminEmail.

[legrego]

@jbudz we are trying to avoid this generalization, since this adds additional complexity when securing access to these settings. We have a similar proposal for xpack:defaultAdminEmail here.

[kobelb]

Could this be generalized to config and admin config? I expect there's a few others, like xpack:defaultAdminEmail.

We might need this long-term, but the difference between this setting and the xPack:defaultAdminEmail is we want anyone to be able opt into telemetry, where-as the xPack:defaultAdminEmail really should only be set by "administrators".

[kobelb]

Also, we could technically leave telemetry alone and allow users to opt into it "per space", even though the stats that would be reported would be for all spaces.

[epixa]

I'm all for this proposal, but let's manage it from xpack_main rather than monitoring.