Feature request: `per_it` template key

[imbolc]

Would you consider adding the {per_it} metric? The meaning of it is to be the opposite of {per_sec}, with a display string like 3s/it.

[djc]

You can define your own with ProgressStyle::with_key().

[keesj]

An example of this can be found here https://github.com/console-rs/indicatif/blob/main/examples/download.rs#L14

I personally need a bit more and would like a throughput to be either in "items/s" or "sec per item" depending on the value. Something like https://gist.github.com/keesj/3bf569a441fee34e9aaf3ff9048ad8c5

This issue can probably be closed