Fix Prometheus metric reporting from cron thread

[danpalmer]

Currently we run 2 threads: API and cron. We are only reporting metrics to Prometheus from the API thread.

https://github.com/prometheus/client_python#multiprocess-mode-gunicorn

This documentation describes how to set up multiprocess support in prometheus_client. It's a little involved.

[danpalmer]

#65

[PeterJCLaw]

@danpalmer did #65 fix this entirely, or is there an outstanding issue here?