mocktracer/mockspan.go
func (s *MockSpan) LogEventWithPayload(event string, payload interface{}) {
	s.LogFields(log.String("event", event), log.Object("payload", payload))
}
func (s *MockSpan) LogEventWithPayload(event string, payload interface{}) {
	s.LogFields(log.String("event", event), log.Object("payload", payload))
}
// LogEventWithPayload belongs to the Span interface