feat: add Prometheus metrics (#439)

* 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

config.yml

@@ -48,6 +48,9 @@ http.port: 6157
 # Enable or disable git operations (clone, pull, push) via HTTP (either `true` or `false`). Default: true
 http.git-enabled: true
 
+# Enable or disable the metrics endpoint (either `true` or `false`). Default: false
+metrics.enabled: false
+
 # SSH built-in server configuration
 # Note: it is not using the SSH daemon from your machine (yet)