feat(exporter): add labels and duration

[jkoelker]

• Adds the speedtest result id as a label to the metric.
• Adds test duration metric.

[jkoelker]

Just traced back the git history of the previous labels and figured out that caching makes the labeling duplicate for those that use it. I'll re-implement with a custom collector that will cache itself.

[jkoelker]

closing in favor of #176