[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/public/plugin/hooks.go

@@ -54,6 +54,7 @@ const (
 	UserHasBeenDeactivatedID                  = 36
 	MessageHasBeenDeletedID                   = 37
 	MessagesWillBeConsumedID                  = 38
+	ServeMetricsID                            = 39
 	TotalHooksID                              = iota
 )
 
@@ -322,4 +323,11 @@ type Hooks interface {
 	//
 	// Minimum server version: 9.1
 	UserHasBeenDeactivated(c *Context, user *model.User)
+
+	// ServeMetrics allows plugins to expose their own metrics endpoint through
+	// the server's metrics HTTP listener (e.g. "localhost:8067").
+	// Requests destined to the /plugins/{id}/metrics path will be routed to the plugin.
+	//
+	// Minimum server version: 9.2
+	ServeMetrics(c *Context, w http.ResponseWriter, r *http.Request)
 }