Support alerts from external Prometheus servers
We should support alerts from external Prometheus servers. We already support receiving alerts from the GitLab deployed Prometheus instance, and we should expand our support to self-managed Prometheus installs as well.
Proposal
This should be relatively easy to do:
In the Prometheus integration settings page (provided we do this in 11.8 and not 12.0, after which it would be under the Settings->Operations->Metrics section)
- Add a new section titled something like
External Prometheus Alerts - Display the URL and an authentication key
- (Nice to have) A button to rotate the key. We may need to use a unique key to avoid breaking GitLab provisioned alerts.
- Provide a link to the documentation, which would have examples on how to setup the Prometheus configuration
For the initial support, we don't need to worry about showing them on the dashboard.
Design
