dnsdist: Add HELP and TYPE for dnsdist_server_ stats

[pieterlexis]

Short description

These were missing from the output. I've put them above the for loop. According to the Open Metrics standard, metrics don't have to be directly underneath the HELP or TYPE (and only one for each metric is allowed).

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)