add a prometheus-text-file-exporter parameter

## Have you checked borgbackup docs, FAQ, and open GitHub issues?

Yes

## Is this a BUG / ISSUE report or a QUESTION?

Feature request

## Describe the problem you're observing.

when running borg it is only possible to parse the output from `--stats` to get a more deeper look into metrics.

It would be nice to have an parameter `--prometheus-exporter ` to write all the metrics to the file. This way we do not need to run any additional command and have it persistent on the disk.