[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/metrics.go

@@ -0,0 +1,54 @@
+// 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 metrics
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+type Module struct {
+	handler http.Handler
+}
+
+func New() *Module {
+	// Use our own gzip handler.
+	opts := promhttp.HandlerOpts{
+		DisableCompression: true,
+	}
+
+	// Instrument handler itself.
+	handler := promhttp.InstrumentMetricHandler(
+		prometheus.DefaultRegisterer,
+		promhttp.HandlerFor(prometheus.DefaultGatherer, opts),
+	)
+
+	return &Module{
+		handler: handler,
+	}
+}
+
+func (m *Module) Route(attachHandler func(method string, path string, f ...gin.HandlerFunc) gin.IRoutes) {
+	attachHandler(http.MethodGet, "", func(c *gin.Context) {
+		// Defer all "/metrics" handling to prom.
+		m.handler.ServeHTTP(c.Writer, c.Request)
+	})
+}