use LinkedTo rather than ChildOf for PublicEndpoint (open-telemetry#272)

This causes us to no longer emit missing root spans if we do not have the trace associated with the tracing headers we receive on public endpoints.

api/trace/api.go

@@ -103,6 +103,7 @@ type SpanOption func(*SpanOptions)
 type SpanOptions struct {
 	Attributes []core.KeyValue
 	StartTime  time.Time
+	Links      []Link
 	Relation   Relation
 	Record     bool
 	SpanKind   SpanKind
@@ -197,6 +198,13 @@ func FollowsFrom(sc core.SpanContext) SpanOption {
 	}
 }
 
+// LinkedTo allows instantiating a Span with initial Links.
+func LinkedTo(sc core.SpanContext, attrs ...core.KeyValue) SpanOption {
+	return func(o *SpanOptions) {
+		o.Links = append(o.Links, Link{sc, attrs})
+	}
+}
+
 // WithSpanKind specifies the role a Span on a Trace.
 func WithSpanKind(sk SpanKind) SpanOption {
 	return func(o *SpanOptions) {

plugin/othttp/handler.go

@@ -150,11 +150,9 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	if sc.IsValid() { // not a valid span context, so no link / parent relationship to establish
 		var opt trace.SpanOption
 		if h.public {
-			// TODO: If the endpoint is a public endpoint, it should start a new trace
-			// and incoming remote sctx should be added as a link
-			// (WithLinks(links...), this option doesn't exist yet). Replace ChildOf
-			// below with something like: opt = trace.WithLinks(sc)
-			opt = trace.ChildOf(sc)
+			// If the endpoint is a public endpoint, it should start a new trace
+			// and incoming remote sctx should be added as a link.
+			opt = trace.LinkedTo(sc)
 		} else { // not a private endpoint, so assume child relationship
 			opt = trace.ChildOf(sc)
 		}

sdk/trace/tracer.go

@@ -60,6 +60,10 @@ func (tr *tracer) Start(ctx context.Context, name string, o ...apitrace.SpanOpti
 
 	spanName := tr.spanNameWithPrefix(name)
 	span := startSpanInternal(tr, spanName, parent, remoteParent, opts)
+	for _, l := range opts.Links {
+		span.AddLink(l)
+	}
+
 	span.tracer = tr
 
 	if span.IsRecording() {