You need to have selected at least one asset-related event before you can select metadata here.
Metadata is not logged by default. Metadata fields and values are only logged if the metadata has changed. The Metadata edit, Workflow, or Upload events can cause metadata to change. We recommend selecting the metadata fields that are likely to change with an event.

Metadata fields configured for logging (for activity types with asset information) are always included. For example, if the Title metadata field is included in the metadata to be logged, then the asset's title is included in the export.

Note: To log metadata events that occur as part of a workflow, for example, metadata edits and macros from actions and markers, you must select the workflow event.

Metadata markers on ingestion (copy, move, or upload) are never logged.

1. From the Tools menu (cogwheel icon), go to Site Configuration > Activity Exports.
2. Open the Metadata tab. 
   
    
3. Here, you can select the metadata to log for every event with asset data. You can filter the displayed fields by selecting a value from the drop-down list. Alternatively, you can search for metadata fields by entering text in the Search field. 
   Select the events you would like to include.
   Tip: To select all metadata fields; select the first event, hold down the Shift key, and select the last event. All events are selected.
4. Select Save.