Change View Attribute Filter to detect if not set.

sdk/metric/view/view.go

@@ -89,14 +89,17 @@ func (v View) TransformInstrument(inst Instrument) (transformed Instrument, matc
 	return inst, true
 }
 
-// TransformAttributes filters an attribute set to the keys in the View. If no
-// filter was provided the original set is returned.
-func (v View) TransformAttributes(input attribute.Set) attribute.Set {
+// AttributeFilter returns a function that returns only attributes specified by
+// WithFilterAttributes.
+// If no filter was provided nil is returned.
+func (v View) AttributeFilter() func(attribute.Set) attribute.Set {
 	if v.filter == nil {
-		return input
+		return nil
+	}
+	return func(input attribute.Set) attribute.Set {
+		out, _ := input.Filter(v.filter)
+		return out
 	}
-	out, _ := input.Filter(v.filter)
-	return out
 }
 
 // TODO: Provide Transform* for AggregationKind (#2816)

sdk/metric/view/view_test.go

@@ -309,7 +309,32 @@ func TestViewMatchName(t *testing.T) {
 	}
 }
 
-func TestViewTransformAttributes(t *testing.T) {
+func TestViewAttributeFilterNoFilter(t *testing.T) {
+	v, err := New(
+		MatchInstrumentName("*"),
+	)
+	require.NoError(t, err)
+	filter := v.AttributeFilter()
+	assert.Nil(t, filter)
+
+	v, err = New(
+		MatchInstrumentName("*"),
+		WithFilterAttributes(),
+	)
+	require.NoError(t, err)
+	filter = v.AttributeFilter()
+	assert.Nil(t, filter)
+
+	v, err = New(
+		MatchInstrumentName("*"),
+		WithFilterAttributes([]attribute.Key{}...),
+	)
+	require.NoError(t, err)
+	filter = v.AttributeFilter()
+	assert.Nil(t, filter)
+}
+
+func TestViewAttributeFilter(t *testing.T) {
 	inputSet := attribute.NewSet(
 		attribute.String("foo", "bar"),
 		attribute.Int("power-level", 9001),
@@ -321,11 +346,6 @@ func TestViewTransformAttributes(t *testing.T) {
 		filter []attribute.Key
 		want   attribute.Set
 	}{
-		{
-			name:   "empty should match all",
-			filter: []attribute.Key{},
-			want:   inputSet,
-		},
 		{
 			name: "Match 1",
 			filter: []attribute.Key{
@@ -371,8 +391,10 @@ func TestViewTransformAttributes(t *testing.T) {
 				WithFilterAttributes(tt.filter...),
 			)
 			require.NoError(t, err)
+			filter := v.AttributeFilter()
+			require.NotNil(t, filter)
 
-			got := v.TransformAttributes(inputSet)
+			got := filter(inputSet)
 			assert.Equal(t, got.Equivalent(), tt.want.Equivalent())
 		})
 	}