tracer.go
func NewTracerFromRequest(r *http.Request, name string) *Tracer {
	span, _ := opentracing.StartSpanFromContext(r.Context(), name)
	return NewTracer(span)
}
func NewTracerFromRequest(r *http.Request, name string) *Tracer {
	span, _ := opentracing.StartSpanFromContext(r.Context(), name)
	return NewTracer(span)
}
// NewTracerFromRequest returns a new tracer that has a root span derived from
// the specified request. A span previously added to the request context using
// Context is automatically used as the parent.