Do not cache reflect.ValueOf() in metrics Labels (#649)

* Benchmark label iterator

* Remove cachedValue, a minor optimization

sdk/metric/benchmark_test.go

@@ -554,3 +554,38 @@ func BenchmarkBatchRecord_8Labels_4Instruments(b *testing.B) {
 func BenchmarkBatchRecord_8Labels_8Instruments(b *testing.B) {
 	benchmarkBatchRecord8Labels(b, 8)
 }
+
+// LabelIterator
+func BenchmarkLabelIterator(b *testing.B) {
+	const labelCount = 1024
+	ctx := context.Background()
+	fix := newFixture(b)
+
+	var rec export.Record
+	fix.pcb = func(_ context.Context, processRec export.Record) error {
+		rec = processRec
+		return nil
+	}
+
+	keyValues := makeLabels(labelCount)
+	counter := fix.meter.NewInt64Counter("test.counter")
+	counter.Add(ctx, 1, keyValues...)
+
+	fix.sdk.Collect(ctx)
+
+	b.ResetTimer()
+
+	labels := rec.Labels()
+	iter := labels.Iter()
+	var val core.KeyValue
+	for i := 0; i < b.N; i++ {
+		if !iter.Next() {
+			iter = labels.Iter()
+			iter.Next()
+		}
+		val = iter.Label()
+	}
+	if false {
+		fmt.Println(val)
+	}
+}

sdk/metric/sdk.go

@@ -92,10 +92,6 @@ type (
 		// the labels, copied into an array of the correct
 		// size for use as a map key.
 		ordered orderedLabels
-
-		// cachedValue contains a `reflect.Value` of the `ordered`
-		// member
-		cachedValue reflect.Value
 	}
 
 	// mapkey uniquely describes a metric instrument in terms of
@@ -174,8 +170,7 @@ var (
 	kvType = reflect.TypeOf(core.KeyValue{})
 
 	emptyLabels = labels{
-		ordered:     [0]core.KeyValue{},
+		ordered: [0]core.KeyValue{},
-		cachedValue: reflect.ValueOf([0]core.KeyValue{}),
 	}
 )
 
@@ -393,13 +388,15 @@ func (m *SDK) makeLabels(kvs []core.KeyValue, sortSlice *sortedLabels) labels {
 // NumLabels is a part of an implementation of the export.LabelStorage
 // interface.
 func (ls *labels) NumLabels() int {
-	return ls.cachedValue.Len()
+	return reflect.ValueOf(ls.ordered).Len()
 }
 
 // GetLabel is a part of an implementation of the export.LabelStorage
 // interface.
 func (ls *labels) GetLabel(idx int) core.KeyValue {
-	return ls.cachedValue.Index(idx).Interface().(core.KeyValue)
+	// Note: The Go compiler successfully avoids an allocation for
+	// the interface{} conversion here:
+	return reflect.ValueOf(ls.ordered).Index(idx).Interface().(core.KeyValue)
 }
 
 // Iter is a part of an implementation of the export.Labels interface.
@@ -459,7 +456,6 @@ func computeOrderedLabels(kvs []core.KeyValue) labels {
 	if ls.ordered == nil {
 		ls.ordered = computeOrderedReflect(kvs)
 	}
-	ls.cachedValue = reflect.ValueOf(ls.ordered)
 	return ls
 }