Investigate / Evaluate whamcloud's lustrefs-exporter

Take a deeper look at https://github.com/whamcloud/lustrefs-exporter

- Does it have all the metrics that we need / want?
- Is it easy to use / extend / whatever?

- It's written in Rust. Not a common language for an exporer. But centainly not a completely bad choice.

If all this turns out go be good, we probably should deprecate this exporer.