Add system metrics & stats

Add repo, svc & rest endpoint for user, role & app metrics:
total, deleted, suspended, archived, ... counters

api/system/spec.json

@@ -1971,5 +1971,23 @@
         }
       }
     ]
+  },
+  {
+    "title": "Statistics",
+    "entrypoint": "stats",
+    "path": "/stats",
+    "authentication": [
+      "Client ID",
+      "Session ID"
+    ],
+    "apis": [
+      {
+        "name": "list",
+        "method": "GET",
+        "title": "List system statistics",
+        "path": "/",
+        "parameters": {}
+      }
+    ]
   }
 ]

api/system/spec/stats.json

@@ -0,0 +1,30 @@
+{
+  "Title": "Statistics",
+  "Interface": "Stats",
+  "Struct": null,
+  "Parameters": null,
+  "Protocol": "",
+  "Authentication": [
+    "Client ID",
+    "Session ID"
+  ],
+  "Path": "/stats",
+  "APIs": [
+    {
+      "Name": "list",
+      "Method": "GET",
+      "Title": "List system statistics",
+      "Path": "/",
+      "Parameters": {
+        "get": [
+          {
+            "name": "metrics",
+            "required": false,
+            "title": "Get only specified metrics",
+            "type": "[]string"
+          }
+        ]
+      }
+    }
+  ]
+}