[chore] add nometrics build tagging to metrics API endpoint (#3331)

* add nometrics API endpoint code, to ensure prometheus NEVER compiled in unless wanted

* whoops, fix build tagging

internal/api/metrics/metrics.go

@@ -15,6 +15,8 @@
 // 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/>.
 
+//go:build !nometrics
+
 package metrics
 
 import (

internal/api/metrics/no_metrics.go

@@ -0,0 +1,31 @@
+// 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/>.
+
+//go:build nometrics
+
+package metrics
+
+import (
+	"github.com/gin-gonic/gin"
+)
+
+type Module struct{}
+
+func New() *Module { return &Module{} }
+
+func (m *Module) Route(attachHandler func(method string, path string, f ...gin.HandlerFunc) gin.IRoutes) {
+}