Allow user to set start options perf HTTP request

Fixes census-instrumentation#869.

plugin/ochttp/client.go

@@ -47,6 +47,10 @@ type Transport struct {
 	// for spans started by this transport.
 	StartOptions trace.StartOptions
 
+	// GetStartOptions allows to set start options per request. If set,
+	// StartOptions is going to be ignored.
+	GetStartOptions func(*http.Request) trace.StartOptions
+
 	// NameFromRequest holds the function to use for generating the span name
 	// from the information found in the outgoing HTTP Request. By default the
 	// name equals the URL Path.
@@ -75,11 +79,17 @@ func (t *Transport) RoundTrip(req *http.Request) (*http.Response, error) {
 	if spanNameFormatter == nil {
 		spanNameFormatter = spanNameFromURL
 	}
+
+	startOpts := t.StartOptions
+	if t.GetStartOptions != nil {
+		startOpts = t.GetStartOptions(req)
+	}
+
 	rt = &traceTransport{
 		base:   rt,
 		format: format,
 		startOptions: trace.StartOptions{
-			Sampler:  t.StartOptions.Sampler,
+			Sampler:  startOpts.Sampler,
 			SpanKind: trace.SpanKindClient,
 		},
 		formatSpanName: spanNameFormatter,

plugin/ochttp/server.go

@@ -56,6 +56,10 @@ type Handler struct {
 	// for spans started by this transport.
 	StartOptions trace.StartOptions
 
+	// GetStartOptions allows to set start options per request. If set,
+	// StartOptions is going to be ignored.
+	GetStartOptions func(*http.Request) trace.StartOptions
+
 	// IsPublicEndpoint should be set to true for publicly accessible HTTP(S)
 	// servers. If true, any trace metadata set on the incoming request will
 	// be added as a linked trace instead of being added as a parent of the
@@ -93,15 +97,21 @@ func (h *Handler) startTrace(w http.ResponseWriter, r *http.Request) (*http.Requ
 		name = h.FormatSpanName(r)
 	}
 	ctx := r.Context()
+
+	startOpts := h.StartOptions
+	if h.GetStartOptions != nil {
+		startOpts = h.GetStartOptions(r)
+	}
+
 	var span *trace.Span
 	sc, ok := h.extractSpanContext(r)
 	if ok && !h.IsPublicEndpoint {
 		ctx, span = trace.StartSpanWithRemoteParent(ctx, name, sc,
-			trace.WithSampler(h.StartOptions.Sampler),
+			trace.WithSampler(startOpts.Sampler),
 			trace.WithSpanKind(trace.SpanKindServer))
 	} else {
 		ctx, span = trace.StartSpan(ctx, name,
-			trace.WithSampler(h.StartOptions.Sampler),
+			trace.WithSampler(startOpts.Sampler),
 			trace.WithSpanKind(trace.SpanKindServer),
 		)
 		if ok {