Fix: Variable item not filtering for item tag (#2006)

Fixes #1930
Fixes #1543 
Fixes #1329 

TODO

- [x] Add test

src/datasource/datasource.ts

@@ -852,7 +852,7 @@ export class ZabbixDatasource extends DataSourceApi<ZabbixMetricsQuery, ZabbixDS
           queryModel.group,
           queryModel.host,
           queryModel.application,
-          null,
+          queryModel.itemTag,
           queryModel.item
         );
         break;