fix(backend): reduce allocations

internal/alertmanager/models.go

@@ -473,8 +473,8 @@ func (am *Alertmanager) ExpiredSilences(labels map[string]string) (silences []*m
 	now := time.Now()
 	maxExpired := now.Add(-config.Config.Silences.Expired)
 	for _, silence := range am.silences {
-		silence := silence
 		if silence.EndsAt.Before(now) && !silence.EndsAt.Before(maxExpired) && silence.IsMatch(labels) {
+			silence := silence
 			silences = append(silences, &silence)
 		}
 	}

internal/mapper/v017/api.go

@@ -50,7 +50,7 @@ func groups(c *client.AlertmanagerAPI, timeout time.Duration, si *intern.Interne
 	ret := make([]models.AlertGroup, 0, len(groups.Payload))
 
 	for _, group := range groups.Payload {
-		ls := models.Labels{}
+		ls := make(models.Labels, 0, len(group.Labels))
 		for k, v := range group.Labels {
 			ls = ls.Set(si.String(k), si.String(v))
 		}
@@ -61,7 +61,7 @@ func groups(c *client.AlertmanagerAPI, timeout time.Duration, si *intern.Interne
 			Alerts:   make(models.AlertList, 0, len(group.Alerts)),
 		}
 		for _, alert := range group.Alerts {
-			ls := models.Labels{}
+			ls := make(models.Labels, 0, len(alert.Labels))
 			for k, v := range alert.Labels {
 				ls = ls.Set(si.String(k), si.String(v))
 			}