[ot bridge] support baggage-only propagation without trace context

Signed-off-by: Chen Xu <chen.x@uber.com>

bridge/opentracing/bridge.go

@@ -732,7 +732,7 @@ func (t *BridgeTracer) Extract(format interface{}, carrier interface{}) (ot.Span
 		bag:             bag,
 		otelSpanContext: trace.SpanContextFromContext(ctx),
 	}
-	if !bridgeSC.otelSpanContext.IsValid() {
+	if !bridgeSC.otelSpanContext.IsValid() && bridgeSC.bag.Len() == 0 {
 		return nil, ot.ErrSpanContextNotFound
 	}
 	return bridgeSC, nil

bridge/opentracing/bridge_test.go

@@ -27,6 +27,7 @@ import (
 	"github.com/opentracing/opentracing-go/ext"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"go.opentelemetry.io/otel/baggage"
 
 	"go.opentelemetry.io/otel"
 	"go.opentelemetry.io/otel/attribute"
@@ -378,6 +379,32 @@ func TestBridgeTracer_ExtractAndInject(t *testing.T) {
 	}
 }
 
+func TestBridgeTracer_ExtractOnlyBaggage(t *testing.T) {
+	testMember, err := baggage.NewMember("key", "val")
+	require.NoError(t, err)
+	testBaggage, err := baggage.New(testMember)
+	require.NoError(t, err)
+
+	customExtract := func(ctx context.Context, carrier propagation.TextMapCarrier) context.Context {
+		return baggage.ContextWithBaggage(ctx, testBaggage)
+	}
+	mockPropagator := &internal.MockTextMapPropagator{ExtractFunc: customExtract}
+
+	bridge := NewBridgeTracer()
+	bridge.SetTextMapPropagator(mockPropagator)
+	spanContext, err := bridge.Extract(ot.TextMap, ot.TextMapCarrier{})
+	assert.NoError(t, err)
+
+	bridgeSpanContext, ok := spanContext.(*bridgeSpanContext)
+	assert.True(t, ok)
+	require.NotNil(t, bridgeSpanContext.otelSpanContext)
+	assert.False(t, bridgeSpanContext.otelSpanContext.IsValid())
+	assert.False(t, bridgeSpanContext.otelSpanContext.IsSampled())
+	assert.Equal(t, 0, bridgeSpanContext.otelSpanContext.TraceState().Len())
+	assert.Equal(t, 1, bridgeSpanContext.bag.Len())
+	assert.Equal(t, testMember, bridgeSpanContext.bag.Member("key"))
+}
+
 type nonDeferWrapperTracer struct {
 	*WrapperTracer
 }

bridge/opentracing/internal/mock.go

@@ -24,6 +24,7 @@ import (
 	"go.opentelemetry.io/otel/attribute"
 	"go.opentelemetry.io/otel/bridge/opentracing/migration"
 	"go.opentelemetry.io/otel/codes"
+	"go.opentelemetry.io/otel/propagation"
 	semconv "go.opentelemetry.io/otel/semconv/v1.17.0"
 	"go.opentelemetry.io/otel/trace"
 )
@@ -292,3 +293,28 @@ func (s *MockSpan) OverrideTracer(tracer trace.Tracer) {
 }
 
 func (s *MockSpan) TracerProvider() trace.TracerProvider { return trace.NewNoopTracerProvider() }
+
+type MockTextMapPropagator struct {
+	FieldsToReturn []string
+	InjectFunc     func(ctx context.Context, carrier propagation.TextMapCarrier)
+	ExtractFunc    func(ctx context.Context, carrier propagation.TextMapCarrier) context.Context
+}
+
+var _ propagation.TextMapPropagator = (*MockTextMapPropagator)(nil)
+
+func (p *MockTextMapPropagator) Inject(ctx context.Context, carrier propagation.TextMapCarrier) {
+	if p.InjectFunc != nil {
+		p.InjectFunc(ctx, carrier)
+	}
+}
+
+func (p *MockTextMapPropagator) Extract(ctx context.Context, carrier propagation.TextMapCarrier) context.Context {
+	if p.ExtractFunc == nil {
+		return ctx
+	}
+	return p.ExtractFunc(ctx, carrier)
+}
+
+func (p *MockTextMapPropagator) Fields() []string {
+	return p.FieldsToReturn
+}