Add new chrony serverstats counters to exposed serverstats metrics

[raphaelthomas]

Makes the exporter compatible with chronyd version 4.4 and later, which return serverstats in format ReplyServerStats4. The additional metrics in said format are added to the serverstats.

Fixes #82.

[raphaelthomas]

Not overly proficient in Go, feedback is much appreciated.

[SuperQ]

I've created a minimal fix PR: https://github.com/SuperQ/chrony_exporter/pull/96.

If you want to work on adding additional stats, that'd be great.

[raphaelthomas]

I've created a minimal fix PR: #96.

If you want to work on adding additional stats, that'd be great.

Wonderful, thanks a lot and apologies for the long silence 🙏🏼 I'll gladly add the missing stats then once #96 is merged.

[SuperQ]

This needs a DCO sign-off. You can use git commit -s --amend to add it.