Generate metric for time between OTK generation and OTK claim

[obrien-j]

This was a request to analyse time to live and time to claim for OTK's as a precursor to further evaluating the expiry duration and whether we could safely reduce the timeout.

Potential enhancement would be to also genetric a metric (new db field) for OTK's that are 'regenerated', which would throw off the eventual metric.

Estimated a day of effort.

This should be done similarly to the metrics for HC.

AC:

• [x] Collect event data.
• [x] Create metric endpoint
• [ ] ~Create a Google Sheet that is updated daily with the metric values.~

[CalvinRodo]

Extractor work is tracked here https://github.com/cds-snc/covid-alert-server-metrics-extractor/issues/2

Closed in favour of broken out task as Prod deployment will not happen until Wednesday December 9th.