Fix excessive allocation in the resource metrics

metrics/resource_view.go

@@ -315,21 +315,45 @@ func resourceToKey(r *resource.Resource) string {
 	if r == nil {
 		return ""
 	}
+
+	// If there are no labels, we have just the Type.
+	if len(r.Labels) == 0 {
+		return r.Type
+	}
+
 	var s strings.Builder
+	writeKV := func(key, value string) { // This lambda doesn't force an allocation.
+		// We use byte values 1 and 2 to avoid colliding with valid resource labels
+		// and to make unpacking easy
+		s.WriteByte('\x01')
+		s.WriteString(key)
+		s.WriteByte('\x02')
+		s.WriteString(value)
+	}
+
+	// If there's only one label, we can skip building and sorting a slice of keys.
+	if len(r.Labels) == 1 {
+		for k, v := range r.Labels {
+			// We know this only executes once.
+			s.Grow(len(r.Type) + len(k) + len(v) + 2)
+			s.WriteString(r.Type)
+			writeKV(k, v)
+			return s.String()
+		}
+	}
+
 	l := len(r.Type)
-	kvs := make([]string, 0, len(r.Labels))
+	keys := make([]string, 0, len(r.Labels))
 	for k, v := range r.Labels {
 		l += len(k) + len(v) + 2
-		// We use byte values 1 and 2 to avoid colliding with valid resource labels
-		// and to make unpacking easy
-		kvs = append(kvs, fmt.Sprintf("\x01%s\x02%s", k, v))
+		keys = append(keys, k)
 	}
 	s.Grow(l)
-	s.WriteString(r.Type)
 
-	sort.Strings(kvs) // Go maps are unsorted, so sort by key to produce stable output.
-	for _, kv := range kvs {
-		s.WriteString(kv)
+	s.WriteString(r.Type)
+	sort.Strings(keys) // Go maps are unsorted, so sort by key to produce stable output.
+	for _, key := range keys {
+		writeKV(key, r.Labels[key])
 	}
 
 	return s.String()

metrics/resource_view_test.go

@@ -228,6 +228,22 @@ func TestResourceAsString(t *testing.T) {
 	}
 }
 
+func BenchmarkResourceToKey(b *testing.B) {
+	for _, count := range []int{0, 1, 5, 10} {
+		labels := make(map[string]string, count)
+		for i := 0; i < count; i++ {
+			labels[fmt.Sprint("key", i)] = fmt.Sprint("value", i)
+		}
+		r := &resource.Resource{Type: "foobar", Labels: labels}
+
+		b.Run(fmt.Sprintf("%d-labels", count), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				resourceToKey(r)
+			}
+		})
+	}
+}
+
 type metricExtract struct {
 	Name   string
 	Labels map[string]string