Allow arbitrary task fields in project statistics filter

apiserver/schema/services/projects.conf

@@ -582,15 +582,9 @@ get_all_ex {
     }
     "2.17": ${get_all_ex."2.16"} {
         request.properties.include_stats_filter {
-            description: The filter for selecting entities that participate in statistics calculation
+            description: The filter for selecting entities that participate in statistics calculation. For each task field that you want to filter on pass the list of allowed values. Prepend the value with '-' to exclude
             type: object
-            properties {
-                system_tags {
-                    description: The list of allowed system tags
-                    type: array
-                    items { type: string }
-                }
-            }
+            additionalProperties: true
         }
     }
 }