vmalert: skip automatically added labels on alerts restore (#871)

Label `alertgroup` was introduced in #611 and automatically added to generated
time series. By mistake, this new label wasn't correctly purged on restore event
and affected alert's ID uniqueness. This commit removes `alertgroup` label
in restore function.

https://github.com/VictoriaMetrics/VictoriaMetrics/issues/870

app/vmalert/alerting.go

@@ -422,6 +422,9 @@ func (ar *AlertingRule) Restore(ctx context.Context, q datasource.Querier, lookb
 			if l.Name == alertNameLabel {
 				continue
 			}
+			if l.Name == alertGroupNameLabel {
+				continue
+			}
 			// drop all overridden labels
 			if _, ok := ar.Labels[l.Name]; ok {
 				continue

app/vmalert/alerting_test.go

@@ -355,6 +355,7 @@ func TestAlertingRule_Restore(t *testing.T) {
 				metricWithValueAndLabels(t, float64(time.Now().Truncate(time.Hour).Unix()),
 					"__name__", alertForStateMetricName,
 					alertNameLabel, "",
+					alertGroupNameLabel, "groupID",
 					"foo", "bar",
 					"namespace", "baz",
 				),