[MM-55268] Implement ServeMetrics plugins hook (#24249)

* Implement ServeMetrics plugins hook

* Update error id

* Simplify

* Revert "Simplify"

This reverts commit c9dc5d5eac6c933ff69e158cf3e34d0973bd3bcd.

* Add comment and error handler

* Wrap error

* Update translation file

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/plugin_api_tests/manual.test_serve_metrics_plugin/main.go

@@ -0,0 +1,27 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package main
+
+import (
+	"net/http"
+
+	"github.com/mattermost/mattermost/server/public/plugin"
+)
+
+type Plugin struct {
+	plugin.MattermostPlugin
+}
+
+func (p *Plugin) ServeMetrics(_ *plugin.Context, w http.ResponseWriter, r *http.Request) {
+	if r.URL.Path == "/subpath" {
+		w.Write([]byte("METRICS SUBPATH"))
+		return
+	}
+
+	w.Write([]byte("METRICS"))
+}
+
+func main() {
+	plugin.ClientMain(&Plugin{})
+}