sub millisecond durations are not recorded with `@opencensus/exporter-ocagent`

[andrewmtoy]

What version of OpenCensus are you using?

0.0.16

What version of Node are you using?

v10.15.1

What did you do?

I created spans that take less than one millisecond and exported them with @opencensus/exporter-ocagent.

What did you expect to see?

I expected to see spans with sub-millisecond durations.

What did you see instead?

Saw span durations of 0 seconds.

Additional context

I believe the bug is on this line:
https://github.com/census-instrumentation/opencensus-node/blob/master/packages/opencensus-core/src/internal/clock.ts#L75

If this.ended is true and this.duration is between 0 and 1 millisecond, the endTime.getTime() returned is the same as this.startTime.getTime() because Date has millisecond resolution.