[distributed sampling] introduced DistributedSampled span member

tracer/span.go

@@ -56,6 +56,11 @@ type Span struct {
 	ParentID uint64             `json:"parent_id"`         // identifier of the span's direct parent
 	Error    int32              `json:"error"`             // error status of the span; 0 means no errors
 	Sampled  bool               `json:"-"`                 // if this span is sampled (and should be kept/recorded) or not
+	// Ideally we would put distributed_sampled on root spans only. But this requires either
+	// writing the marshaller manually or use hacky "hiding" of the field,
+	// see http://attilaolah.eu/2014/09/10/json-and-struct-composition-in-go/ on how to do this.
+	// For now, keep it as is, send it all the time, it's minor network overhead.
+	DistributedSampled bool `json:"distributed_sampled"` // if this span is sampled (as in distributed tracing, all parts must be kept) or not
 
 	sync.RWMutex
 	tracer   *Tracer // the tracer that generated this span
@@ -72,16 +77,17 @@ type Span struct {
 // Most of the time one should prefer the Tracer NewRootSpan or NewChildSpan methods.
 func NewSpan(name, service, resource string, spanID, traceID, parentID uint64, tracer *Tracer) *Span {
 	return &Span{
-		Name:     name,
-		Service:  service,
-		Resource: resource,
-		Meta:     tracer.getAllMeta(),
-		SpanID:   spanID,
-		TraceID:  traceID,
-		ParentID: parentID,
-		Start:    now(),
-		Sampled:  true,
-		tracer:   tracer,
+		Name:               name,
+		Service:            service,
+		Resource:           resource,
+		Meta:               tracer.getAllMeta(),
+		SpanID:             spanID,
+		TraceID:            traceID,
+		ParentID:           parentID,
+		Start:              now(),
+		Sampled:            true,
+		DistributedSampled: true,
+		tracer:             tracer,
 	}
 }