API metrics

This release makes API calls more resilient and adds API error metrics.
Also fixes metrics help texts.

Be quick or be released

Quick release that adds the SKIPPED backup state for Prometheus, showing as warning in Grafana dashbard.

Thou shalt have the API license

This release mainly adds an error message when API doesn't allow to be used because of licensing issues.

Also adds debugging and fixes a typo in the systemd unit file.

Thou shalt test your software

This release adresses an issue where object backup state is incertain if never run...
Thanks to @vitorfulgencio for the catch.

Initial release

So this is our first release of this exporter for Nakivo B&R.
Problably not enough tested (a couple of instances only), but here we are ^^