tracing.go
func TracerFromRegistrar(registrar Registrar) opentracing.Tracer {
	if tracerProvider, ok := registrar.(tracerProvider); ok {
		return tracerProvider.Tracer()
	}
	return opentracing.GlobalTracer()
}
func TracerFromRegistrar(registrar Registrar) opentracing.Tracer {
	if tracerProvider, ok := registrar.(tracerProvider); ok {
		return tracerProvider.Tracer()
	}
	return opentracing.GlobalTracer()
}
// TracerFromRegistrar returns an OpenTracing Tracer embedded in the Registrar,
// assuming that Registrar has a Tracer() method. Otherwise it returns default Global Tracer.