Add additional ntp metrics

[lindhor]

Add metrics similar to node_exporter. I.e. rtt, reference time, root delay/dispersion/distance, precision and leap second indicator.

Also adds server label to stratum for consistency.

closes #25

[lindhor]

I think it’s ready. Please review and provide any feedback.

[majewsky]

Thanks! There was a small copy-paste error that I corrected myself. There will be a release soonish (on the order of days to weeks), we have a recurring task to cut releases on all the small tools that we maintain and this one will be handled as part of that.

[lindhor]

Thanks!!