group_by is ignored

[radon-at-beeper]

Alertmanager has a group_by feature that allows multiple instances of a firing alert to be combined and displayed in a single message, instead of all being sent separately. This is how VictoriaMetrics describes it:

Except that this Matrix bot undoes all the group_by work and sends each firing alert individually again:

matrix-alertmanager/src/routes.js

Line 30 in 5008102

const promises = alerts.map(alert => client.sendAlert(roomId, alert))

Thus rendering the feature useless.

This is related to #35, although supporting fully customizable templates is not a requirement just in order to handle group_by correctly in the built-in template.