[Telemetry] Opt-in/out notifications from server should not crash the Kibana request

[afharo]

Kibana version: 7.9

Describe the bug:

When calling /api/telemetry/v2/optIn, Kibana notifies the remote telemetry service about the user's final decision. If the user opted-out, this will be the very last notification we send to the remote telemetry service.

If config.sendUsageFrom === 'server' but the server does not have access to the telemetry.elastic.co domain, the API call returns 500, not allowing the user to dismiss the telemetry banner.

Expected behavior:

The user's decision should be stored and the server should handle nicely if it can't reach the telemetry service.

[elasticmachine]

Pinging @elastic/kibana-telemetry (Team:KibanaTelemetry)

[elasticmachine]

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