Add readOnly flag to statstic - has no effect (Azure#16286)

Author: docohe

specification/azure-kusto/resource-manager/Microsoft.Kusto/stable/2021-08-27/kusto.json

@@ -4039,6 +4039,7 @@
         },
         "statistics": {
           "$ref": "#/definitions/DatabaseStatistics",
+          "readOnly": true,
           "description": "The statistics of the database."
         },
         "isFollowed": {
@@ -4069,6 +4070,7 @@
         },
         "statistics": {
           "$ref": "#/definitions/DatabaseStatistics",
+          "readOnly": true,
           "description": "The statistics of the database."
         },
         "leaderClusterResourceId": {