Given a time series, is it possible to find the timestamp of the last value change?
For example, a cron job runs every night. It first runs an rsync job, and later computes the size of the target directory. The actual metric value is not all that significant, but I would like to verify that it actually changes every night.
Is it possible to create a query that return a scalar value with the timestamp of the last value change?
No, you can't do that.
For many use cases, there are workarounds. For mine, there isn't.