internal/telemetry: change detach to be an event

deliver detach to the exporter as an event rather than hard coding the
span detach behavior.

Change-Id: I87b6e2a3596fea338908c11ba0b219176b6305bf
Reviewed-on: https://go-review.googlesource.com/c/tools/+/222542
Reviewed-by: Robert Findley <rfindley@google.com>

internal/telemetry/event.go

@@ -16,6 +16,7 @@ const (
 	EventStartSpan
 	EventEndSpan
 	EventTag
+	EventDetach
 )
 
 type Event struct {

internal/telemetry/export/trace.go

@@ -70,16 +70,12 @@ func ContextSpan(ctx context.Context, event telemetry.Event) context.Context {
 		if span := GetSpan(ctx); span != nil {
 			span.Finish = event.At
 		}
+	case telemetry.EventDetach:
+		return context.WithValue(ctx, spanContextKey, nil)
 	}
 	return ctx
 }
 
-// Detach returns a context without an associated span.
-// This allows the creation of spans that are not children of the current span.
-func Detach(ctx context.Context) context.Context {
-	return context.WithValue(ctx, spanContextKey, nil)
-}
-
 func (s *SpanContext) Format(f fmt.State, r rune) {
 	fmt.Fprintf(f, "%v:%v", s.TraceID, s.SpanID)
 }

internal/telemetry/trace/trace.go

@@ -31,5 +31,8 @@ func StartSpan(ctx context.Context, name string, tags ...telemetry.Tag) (context
 // Detach returns a context without an associated span.
 // This allows the creation of spans that are not children of the current span.
 func Detach(ctx context.Context) context.Context {
-	return export.Detach(ctx)
+	return export.ProcessEvent(ctx, telemetry.Event{
+		Type: telemetry.EventDetach,
+		At:   time.Now(),
+	})
 }