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[bot]]

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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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