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

docs/.vitepress/config.mts

@@ -46,6 +46,7 @@ export default defineConfig({
                     {text: 'Custom assets', link: '/custom-assets'},
                     {text: 'Custom links', link: '/custom-links'},
                     {text: 'Cheat Sheet', link: '/cheat-sheet'},
+                    {text: 'Metrics', link: '/metrics'},
                     {text: 'Admin panel', link: '/admin-panel'},
                 ], collapsed: false
             },