labels: avoid lint warning on New()

This code is a bit cleaner.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

model/labels/labels.go

@@ -357,9 +357,7 @@ func EmptyLabels() Labels {
 // The caller has to guarantee that all label names are unique.
 func New(ls ...Label) Labels {
 	set := make(Labels, 0, len(ls))
-	for _, l := range ls {
-		set = append(set, l)
-	}
+	set = append(set, ls...)
 	sort.Sort(set)
 
 	return set