Statistics: Add selected fields in aggregate timeseries metadata

Currently the selected default fields are documented in the API, however for convenience and to avoid bugs adding an array with the currently provided fields might improve the user experience:

fields: ["timestamp", "average_pps_in", "average_pps_out", "average_ops_in", "average_ops_out"],

Also fields that are unavailable could then just be ignored instead of explicitly null ed because there is no data.