While analyzing disk space used by data collected by Metricbeat and stored in indexes with TSDB and synthetic _source enabled (elastic/elasticsearch#85649), @nik9000 found that event.duration takes up to 16.7% of the disk space.

This field is automatically added by Metricbeat, by default, with the duration of the fetch operation.

I guess that the main purpose of this field is to monitor or debug metrics collection itself, but this may not be so useful for the final users of most modules.

Being in Metricbeat, this is also added to metrics documents collected by Agent.

We should reconsider this field, or disable it by default.

cc @ruflin