[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>

2 files changed, 46 insertions, 0 deletions

diff --git a/internal/web/metrics.go b/internal/web/metrics.go
new file mode 100644
index 000000000..eb5530290
--- /dev/null
+++ b/internal/web/metrics.go
@@ -0,0 +1,33 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package web
+
+import (
+	"github.com/gin-gonic/gin"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+const (
+	metricsPath = "/metrics"
+	metricsUser = "metrics"
+)
+
+func (m *Module) metricsGETHandler(c *gin.Context) {
+	h := promhttp.Handler()
+	h.ServeHTTP(c.Writer, c.Request)
+}
diff --git a/internal/web/web.go b/internal/web/web.go
index 86e74d6f8..6a21a754b 100644
--- a/internal/web/web.go
+++ b/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) })