Core fields
The following fields are used by core.
-  $raw_event(type: string)
- 
The data received from stream modules (im_file, im_tcp, etc.). 
-  $EventReceivedTime(type: datetime)
- 
The time when the event is received. The value is not modified if the field already exists. 
-  $Hostname(type: string)
- 
The name of the host where the nxlog is running. The value is not modified if the field already exists. 
-  $SourceModuleName(type: string)
- 
The name of the module instance, for input modules. The value is not modified if the field already exists. 
-  $SourceModuleType(type: string)
- 
The type of module instance (such as im_file), for input modules. The value is not modified if the field already exists.
