func (t *MockTracer) Reset() {
	t.Lock()
	defer t.Unlock()
	t.finishedSpans = []*MockSpan{}
}
func (t *MockTracer) Reset() {
	t.Lock()
	defer t.Unlock()
	t.finishedSpans = []*MockSpan{}
}
// Reset clears the internally accumulated finished spans. Note that any
// extant MockSpans will still append to finishedSpans when they Finish(),
// even after a call to Reset().