Add Tracestate into the SamplingResult struct

Add `trace.Tracestate` field into the SDK `trace.SamplingResult` struct.

Use ParentContext from SamplingParameters to return Tracestate in
`traceIDRatioSampler`, `alwaysOnSampler` and `alwaysOffSampler`.

Add a new test to check that Tracestate is passed.

sdk/trace/sampling.go

@@ -56,10 +56,11 @@ const (
 	RecordAndSample
 )
 
-// SamplingResult conveys a SamplingDecision and a set of Attributes.
+// SamplingResult conveys a SamplingDecision, set of Attributes and a Tracestate.
 type SamplingResult struct {
 	Decision   SamplingDecision
 	Attributes []label.KeyValue
+	Tracestate trace.TraceState
 }
 
 type traceIDRatioSampler struct {
@@ -70,9 +71,15 @@ type traceIDRatioSampler struct {
 func (ts traceIDRatioSampler) ShouldSample(p SamplingParameters) SamplingResult {
 	x := binary.BigEndian.Uint64(p.TraceID[0:8]) >> 1
 	if x < ts.traceIDUpperBound {
-		return SamplingResult{Decision: RecordAndSample}
+		return SamplingResult{
+			Decision:   RecordAndSample,
+			Tracestate: p.ParentContext.TraceState,
+		}
+	}
+	return SamplingResult{
+		Decision:   Drop,
+		Tracestate: p.ParentContext.TraceState,
 	}
-	return SamplingResult{Decision: Drop}
 }
 
 func (ts traceIDRatioSampler) Description() string {
@@ -102,7 +109,10 @@ func TraceIDRatioBased(fraction float64) Sampler {
 type alwaysOnSampler struct{}
 
 func (as alwaysOnSampler) ShouldSample(p SamplingParameters) SamplingResult {
-	return SamplingResult{Decision: RecordAndSample}
+	return SamplingResult{
+		Decision:   RecordAndSample,
+		Tracestate: p.ParentContext.TraceState,
+	}
 }
 
 func (as alwaysOnSampler) Description() string {
@@ -120,7 +130,10 @@ func AlwaysSample() Sampler {
 type alwaysOffSampler struct{}
 
 func (as alwaysOffSampler) ShouldSample(p SamplingParameters) SamplingResult {
-	return SamplingResult{Decision: Drop}
+	return SamplingResult{
+		Decision:   Drop,
+		Tracestate: p.ParentContext.TraceState,
+	}
 }
 
 func (as alwaysOffSampler) Description() string {

sdk/trace/sampling_test.go

@@ -22,6 +22,7 @@ import (
 
 	"github.com/stretchr/testify/require"
 
+	"go.opentelemetry.io/otel/label"
 	"go.opentelemetry.io/otel/trace"
 )
 
@@ -189,3 +190,47 @@ func TestTraceIdRatioSamplesInclusively(t *testing.T) {
 		}
 	}
 }
+
+func TestTracestateIsPassed(t *testing.T) {
+	testCases := []struct {
+		name    string
+		sampler Sampler
+	}{
+		{
+			"notSampled",
+			NeverSample(),
+		},
+		{
+			"sampled",
+			AlwaysSample(),
+		},
+		{
+			"parentSampled",
+			ParentBased(AlwaysSample()),
+		},
+		{
+			"parentNotSampled",
+			ParentBased(NeverSample()),
+		},
+		{
+			"traceIDRatioSampler",
+			TraceIDRatioBased(.5),
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			traceState, err := trace.TraceStateFromKeyValues(label.String("k", "v"))
+			if err != nil {
+				t.Error(err)
+			}
+
+			parentCtx := trace.SpanContext{
+				TraceState: traceState,
+			}
+			params := SamplingParameters{ParentContext: parentCtx}
+
+			require.Equal(t, traceState, tc.sampler.ShouldSample(params).Tracestate, "traceState is not equal")
+		})
+	}
+}