tracer.go
func (t *Tracer) Pop() {
	// check list
	if len(t.spans) == 0 {
		return
	}

	// finish last span
	t.Last().Finish()

	// resize slice
	t.spans = t.spans[:len(t.spans)-1]
}
func (t *Tracer) Pop() {
	// check list
	if len(t.spans) == 0 {
		return
	}

	// finish last span
	t.Last().Finish()

	// resize slice
	t.spans = t.spans[:len(t.spans)-1]
}
// Pop finishes and removes the last pushed span.