dnsdist: Add more stats about DoH HTTP responses

[rgacogne]

Short description

Add HTTP status code statistics, per HTTP version, and publish all the DoH stats to the API and prometheus.
While most metrics refer only to valid DNS queries, the XXXResponses ones refer to all responses, even those made to invalid HTTP queries.

Closes #7898.

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)