tracer.go
func NewTracer(root opentracing.Span) *Tracer {
	return &Tracer{
		root:  root,
		spans: make([]opentracing.Span, 0, 32),
	}
}
func NewTracer(root opentracing.Span) *Tracer {
	return &Tracer{
		root:  root,
		spans: make([]opentracing.Span, 0, 32),
	}
}
// NewTracer returns a new tracer with the specified root span.