expose metrics for scraping via prometheus at /metrics
- localhost only
- expose statistics like "users with 2fa" "total users" etc.
- see https://prometheus.io/docs/practices/naming/ for naming schema
- see https://prometheus.io/docs/instrumenting/writing_exporters/ for howto write an exporter
- see https://github.com/prometheus/docs/blob/master/content/docs/instrumenting/exposition_formats.md for format