Reverse order of attribute precedence when merging two Resources (#1501)

* Reverse order of attribute precedence when merging two Resources

This reflects a change in the specification itself.
open-telemetry/opentelemetry-specification#1345

* Resolves #1500

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>

CHANGELOG.md

@@ -8,6 +8,10 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ## [Unreleased]
 
+### Changed
+
+- Reverse order in which `Resource` attributes are merged, per change in spec. (#1501)
+
 ## [0.16.0] - 2020-01-13
 
 ### Added

sdk/resource/resource.go

@@ -84,7 +84,8 @@ func (r *Resource) Equal(eq *Resource) bool {
 // Merge creates a new resource by combining resource a and b.
 //
 // If there are common keys between resource a and b, then the value
-// from resource a is preserved.
+// from resource b will overwrite the value from resource a, even
+// if resource b's value is empty.
 func Merge(a, b *Resource) *Resource {
 	if a == nil && b == nil {
 		return Empty()
@@ -96,9 +97,9 @@ func Merge(a, b *Resource) *Resource {
 		return a
 	}
 
-	// Note: 'a' labels will overwrite 'b' with last-value-wins in label.Key()
-	// Meaning this is equivalent to: append(b.Attributes(), a.Attributes()...)
-	mi := label.NewMergeIterator(a.LabelSet(), b.LabelSet())
+	// Note: 'b' labels will overwrite 'a' with last-value-wins in label.Key()
+	// Meaning this is equivalent to: append(a.Attributes(), b.Attributes()...)
+	mi := label.NewMergeIterator(b.LabelSet(), a.LabelSet())
 	combine := make([]label.KeyValue, 0, a.Len()+b.Len())
 	for mi.Next() {
 		combine = append(combine, mi.Label())

sdk/resource/resource_test.go

@@ -32,6 +32,7 @@ var (
 	kv21 = label.String("k2", "v21")
 	kv31 = label.String("k3", "v31")
 	kv41 = label.String("k4", "v41")
+	kv42 = label.String("k4", "")
 )
 
 func TestNew(t *testing.T) {
@@ -91,13 +92,13 @@ func TestMerge(t *testing.T) {
 			name: "Merge with common key order1",
 			a:    resource.NewWithAttributes(kv11),
 			b:    resource.NewWithAttributes(kv12, kv21),
-			want: []label.KeyValue{kv11, kv21},
+			want: []label.KeyValue{kv12, kv21},
 		},
 		{
 			name: "Merge with common key order2",
 			a:    resource.NewWithAttributes(kv12, kv21),
 			b:    resource.NewWithAttributes(kv11),
-			want: []label.KeyValue{kv12, kv21},
+			want: []label.KeyValue{kv11, kv21},
 		},
 		{
 			name: "Merge with common key order4",
@@ -135,6 +136,18 @@ func TestMerge(t *testing.T) {
 			b:    nil,
 			want: []label.KeyValue{kv11},
 		},
+		{
+			name: "Merge with first resource value empty string",
+			a:    resource.NewWithAttributes(kv42),
+			b:    resource.NewWithAttributes(kv41),
+			want: []label.KeyValue{kv41},
+		},
+		{
+			name: "Merge with second resource value empty string",
+			a:    resource.NewWithAttributes(kv41),
+			b:    resource.NewWithAttributes(kv42),
+			want: []label.KeyValue{kv42},
+		},
 	}
 	for _, c := range cases {
 		t.Run(fmt.Sprintf("case-%s", c.name), func(t *testing.T) {