* feat: add Prometheus metrics
* setup metrics using Prometheus client under /metrics endpoint
* add configuration value for metrics
* configure Prometheus middleware for generic metrics
* provide metrics for totals of users, gists and SSH keys
* modify test request to optionally return the response
* provide integration test for Prometheus metrics
* update documentation
* chore: make fmt
* Skip CSRF for embeds
The CSRF middleware sets a _csrf cookie also for loading the embed
javascript on third-party sites. With this change no _csrf cookie is set
when loading the embed javascript (regardless if third-party site or
first-party).
