refactor: add some more empty report set objects

for efficiency and consistency

report/controls.go

@@ -56,11 +56,11 @@ type NodeControls struct {
 	Controls  StringSet
 }
 
+var emptyNodeControls = NodeControls{Controls: MakeStringSet()}
+
 // MakeNodeControls makes a new NodeControls
 func MakeNodeControls() NodeControls {
-	return NodeControls{
-		Controls: MakeStringSet(),
-	}
+	return emptyNodeControls
 }
 
 // Copy is a noop, as NodeControls is immutable

report/metrics.go

@@ -65,11 +65,13 @@ func MakeSingletonMetric(t time.Time, v float64) Metric {
 
 }
 
+var emptyMetric = Metric{}
+
 // MakeMetric makes a new Metric from unique samples incrementally ordered in
 // time.
 func MakeMetric(samples []Sample) Metric {
 	if len(samples) < 1 {
-		return Metric{}
+		return emptyMetric
 	}
 
 	var (