globaltracer.go
func StartSpan(operationName string, opts ...StartSpanOption) Span {
	return globalTracer.tracer.StartSpan(operationName, opts...)
}
func StartSpan(operationName string, opts ...StartSpanOption) Span {
	return globalTracer.tracer.StartSpan(operationName, opts...)
}
// StartSpan defers to `Tracer.StartSpan`. See `GlobalTracer()`.