Add baggage ForEach to avoid copies, and a test

baggage/baggage.go

@@ -36,6 +36,12 @@ func Set(ctx context.Context) label.Set {
 	return label.NewSet(values...)
 }
 
+// ForEach allows visiting the baggage values in a context without
+// copying a slice.
+func ForEach(ctx context.Context, f func(kv label.KeyValue) bool) {
+	baggage.MapFromContext(ctx).Foreach(f)
+}
+
 // Value returns the value related to key in the baggage of ctx. If no
 // value is set, the returned label.Value will be an uninitialized zero-value
 // with type INVALID.

baggage/baggage_test.go

@@ -18,6 +18,8 @@ import (
 	"context"
 	"testing"
 
+	"github.com/stretchr/testify/require"
+
 	"go.opentelemetry.io/otel/internal/baggage"
 	"go.opentelemetry.io/otel/label"
 )
@@ -84,3 +86,22 @@ func TestBaggage(t *testing.T) {
 		t.Fatal("WithoutBaggage failed to clear baggage")
 	}
 }
+
+func TestForEach(t *testing.T) {
+	ctx := ContextWithValues(
+		context.Background(),
+		label.String("A", "B"),
+		label.String("C", "D"),
+	)
+
+	out := map[string]string{}
+	ForEach(ctx, func(kv label.KeyValue) bool {
+		out[string(kv.Key)] = kv.Value.Emit()
+		return true
+	})
+
+	require.EqualValues(t, map[string]string{
+		"A": "B",
+		"C": "D",
+	}, out)
+}