[chore/bugfix] Fix double gzip on prometheus endpoint (#2383)

* [chore] Move "/metrics" into separate API module

* use our own gzip middleware for prom

internal/api/metrics.go

@@ -0,0 +1,66 @@
+// 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 api
+
+import (
+	"github.com/gin-gonic/gin"
+	"github.com/superseriousbusiness/gotosocial/internal/api/metrics"
+	"github.com/superseriousbusiness/gotosocial/internal/config"
+	"github.com/superseriousbusiness/gotosocial/internal/middleware"
+	"github.com/superseriousbusiness/gotosocial/internal/router"
+)
+
+type Metrics struct {
+	metrics *metrics.Module
+}
+
+func (mt *Metrics) Route(r *router.Router, m ...gin.HandlerFunc) {
+	if !config.GetMetricsEnabled() {
+		// Noop: metrics
+		// not enabled.
+		return
+	}
+
+	// Create new group on top level "metrics" prefix.
+	metricsGroup := r.AttachGroup("metrics")
+	metricsGroup.Use(m...)
+	metricsGroup.Use(
+		middleware.CacheControl(middleware.CacheControlConfig{
+			// Never cache metrics responses.
+			Directives: []string{"no-store"},
+		}),
+	)
+
+	// Attach basic auth if enabled.
+	if config.GetMetricsAuthEnabled() {
+		var (
+			username = config.GetMetricsAuthUsername()
+			password = config.GetMetricsAuthPassword()
+			accounts = gin.Accounts{username: password}
+		)
+		metricsGroup.Use(gin.BasicAuth(accounts))
+	}
+
+	mt.metrics.Route(metricsGroup.Handle)
+}
+
+func NewMetrics() *Metrics {
+	return &Metrics{
+		metrics: metrics.New(),
+	}
+}