Add MetricsBackend with prometheus counter support

src/server/mods.cpp

@@ -23,6 +23,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "scripting_server.h"
 #include "content/subgames.h"
 #include "porting.h"
+#include "util/metricsbackend.h"
 
 /**
  * Manage server mods

src/server/mods.h

@@ -20,7 +20,10 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #pragma once
 
 #include "content/mods.h"
+#include <memory>
 
+class MetricsBackend;
+class MetricCounter;
 class ServerScripting;
 
 /**