[Downsampling] Extend the default_metric of the aggregate_metric_double to support more values #128356
Description
Description
Currently, when downsampling a gauge field we keep the aggregate value in a new type aggregate_metric_double which consists of min, max, sum and value_count. This type supports accurately certain aggregations but not all, see here for more info.
In order to support aggregations like exists, range, term, terms it uses the default_metric which picks one of the available values to use.
We would like to extend the default_metric options to include the last measurement and/or the average.