refactor(metrics): use instance-based design

[danroc]

Move all Prometheus collectors and registry from package-level globals into PrometheusCollector struct fields. Initialization moves from init() to NewCollector(), Handler() becomes a method, and Reset() is removed since tests now create isolated collector instances.

[codecov]

Codecov Report

❌ Patch coverage is 99.18699% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.00%. Comparing base (707df91) to head (1ecab3b).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/geoblock/main.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #470      +/-   ##
==========================================
+ Coverage   90.08%   91.00%   +0.91%     
==========================================
  Files          12       12              
  Lines         686      756      +70     
==========================================
+ Hits          618      688      +70     
  Misses         62       62              
  Partials        6        6              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.