* 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
* Add a setting to allow accessing individual gists without auth
This is a middle ground between the existing setting "Require Login",
which requires login to do anything at all, and having it off, which
shows a public list of gists and more generally allows discovering info
about the users/gists of the instance without login.
The idea of this setting is that it is "require login" for everything
except individual gists.
Fixes#228.
Co-authored-by: Thomas Miceli <tho.miceli@gmail.com>
