Alerting: Feature toggle to disallow sending alerts externally

[stevesg]

What is this feature?

Adds a feature toggle to disallow sending alerts to external Alertmanager datasources.

• If a user already has an admin configuration, this is ignored and a warning logged.
• If a user tries to write an admin configuration to send to external Alertmanagers, it is rejected.

Note that when reading the admin configuration, the persisted configuration is returned. I'm not entirely convinced this is correct, perhaps we should return the effective configuration.

Note this commit does not include frontend changes. We will likely want to modify the Settings page to not show the option to configure external Alertmanagers.

Why do we need this feature?

Site policy may dictate that alerts should be handled only internally, or if they are sent externally, should be done via the new Remote Alertmanager feature.

Note: There is no intention to remove the ability to send externally, this is merely being provided as an option for site administrators who want to disallow it.

Who is this feature for?

Site administrators of Grafana instances.

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[santihernandezc]

LGTM 👍

I got the warning log line but the UI still shows the selected option, which is confusing for users unaware of the feature toggle being active.

WARN [05-21|16:21:23] Alertmanager choice in configuration will be ignored due to feature flags logger=ngalert.sender.router org=1 choice=all

The admin config is not modified in the database, which is ok by me, especially if someone wants to disable this feature toggle and get alerts routed to external Alertmanagers without having to re-configure that routing.

I created an issue for hiding the "Send alerts to" options in the front end.