remove empty labels from fingerprint (#39)

Co-authored-by: Erez Levi <[email protected]>

pkg/services/ngalert/state/historian/loki.go

@@ -325,7 +325,14 @@ func StatesToStream(rule history_model.RuleMeta, states []state.StateTransition,
 
 func calculateFingerprint(labels data.Labels) string {
 	cpLabels := labels.Copy()
-	delete(cpLabels, "__alert_rule_namespace_uid__")
+	for k, v := range cpLabels {
+		// The Grafana Alertmanager skips empty and namespace UID labels.
+		// To get the same alert fingerprint we need to remove these labels too.
+		// https://github.com/grafana/alerting/blob/2dda1c67ec02625ac9fc8607157b3d5825d47919/notify/grafana_alertmanager.go#L722-L724
+		if len(v) == 0 || k == "__alert_rule_namespace_uid__" {
+			delete(cpLabels, k)
+		}
+	}
 	return labelFingerprint(cpLabels)
 }