[feature] Initial Prometheus metrics implementation (#2334)

* feat: Initial OTEL metrics

* docs: add metrics documentation

* fix: metrics endpoint conditional check

* feat: metrics endpoint basic auth

* fix: make metrics-auth-enabled default false

* fix: go fmt helpers.gen.go

* fix: add metric-related env vars to envparsing.sh

* fix: metrics docs

* fix: metrics related stuff in envparsing.sh

* fix: metrics docs

* chore: metrics docs wording

* fix: metrics stuff in envparsing?

* bump otel versions

---------

Co-authored-by: Tsuribori <user@acertaindebian>
Co-authored-by: Tsuribori <none@example.org>
Co-authored-by: tsmethurst <tobi.smethurst@protonmail.com>

internal/web/web.go

@@ -110,6 +110,19 @@ func (m *Module) Route(r *router.Router, mi ...gin.HandlerFunc) {
 	r.AttachHandler(http.MethodGet, domainBlockListPath, m.domainBlockListGETHandler)
 	r.AttachHandler(http.MethodGet, tagsPath, m.tagGETHandler)
 
+	// Prometheus metrics export endpoint
+	if config.GetMetricsEnabled() {
+		metricsGroup := r.AttachGroup(metricsPath)
+		metricsGroup.Use(mi...)
+		// Attach basic auth if enabled
+		if config.GetMetricsAuthEnabled() {
+			metricsGroup.Use(gin.BasicAuth(gin.Accounts{
+				config.GetMetricsAuthUsername(): config.GetMetricsAuthPassword(),
+			}))
+		}
+		metricsGroup.Handle(http.MethodGet, "", m.metricsGETHandler)
+	}
+
 	// Attach redirects from old endpoints to current ones for backwards compatibility
 	r.AttachHandler(http.MethodGet, "/auth/edit", func(c *gin.Context) { c.Redirect(http.StatusMovedPermanently, userPanelPath) })
 	r.AttachHandler(http.MethodGet, "/user", func(c *gin.Context) { c.Redirect(http.StatusMovedPermanently, userPanelPath) })