New feature: Manually fire alerts to test connectivity to target

[henrik-caesar]

Hello,
based on my experience as user of other systems that support alerting, and also as the Product Manager for two such systems, I suggest the following improvement to the Alertmanager (reservations for my limited knowledge about the implementation and use of Prometheus alerting).
*Add a feature that allows the user to trigger/fire an alert from inside the Alertmanager UI.

• It must be possible to select any target that is configured.
• If possible use a mock alert configuration, for message, tags etc.
• If possible, trigger select an actual configured alert
• If possible, log and make available any return messages, errors etc. for trouble shooting

If unclear, I hope I have at least made the principles for the feature understood.

/Henrik