Let's say I want to duplicate a label, such that I want to keep the existing label in the metric and also add another label to the metric with another name. It would be similar to 'join' but without joining the values of two other labels into one, it would be a straight copy of a label.
The reason this question came up was that we have older dashboards and metrics that we have been keeping long term, so we want the long term data to be searchable with the old dashboards (but to do so, the data would need to be 'updated' to include a label that isn't already used (the use of the world 'aggregate' was heavily used and harvest uses the abbreviated version of aggr instead, so queries looking for aggregate would fail without including it in the new metrics.