Add prometheus exporter

[bsquizz]

Expose metrics that we care about:

Request latency
API call requests/errors
Amount of tasks scheduled and/or received/processed by worker
Tasks succeeded vs tasks failed

[rsnyman]

Something like this might be useful: https://pypi.org/project/flask-prometheus-metrics/

[rsnyman]

There's also this, which has the option of adding custom points: https://pypi.org/project/prometheus-flask-exporter/