Simplest way to add subsegment to a NodeJS Lambda.

Hi!

Shouldn't this work?

```javascript
export const handler = async () => {
const segment = AWSXRay.getSegment();
const subSegment = segment.addNewSubsegment('ExternalCall');
// do stuff...
subSegment.close();
}
```

When debugging, I'm getting this: `Ignoring flush on subsegment 846f3500400178a9. Associated segment is marked as not sampled.`

And, most importantly, nothing gets registered or displayed on XRay.