core/validatorapi: metrics for user-agent and content-type

[pinebit]

Added two new metrics to VAPI:

• core_validatorapi_request_total
• core_validatorapi_vc_user_agent

These will allows us to track VC's User-Agent strings to determine VC vendor/version implicitly.
And the total number of requests have Content-Type label to track requests' format made by VC.

category: feature
ticket: #3107

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.81%. Comparing base (278bcf7) to head (2e7c2fd).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3112      +/-   ##
==========================================
- Coverage   56.82%   56.81%   -0.01%     
==========================================
  Files         208      208              
  Lines       29153    29159       +6     
==========================================
+ Hits        16566    16568       +2     
- Misses      10749    10752       +3     
- Partials     1838     1839       +1     

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