fix(Monitoring): Serve /metrics on a dedicated port

[emyller]

Reduce metrics loss under load by serving Prometheus metrics on a dedicated process and port, independent of the main API/task-processor server.

Changes

• Add standalone Prometheus metrics HTTP server on port 9100
• Start server via gunicorn when_ready hook when PROMETHEUS_ENABLED=true
• Add integration tests validating equivalence with Django /metrics endpoint

This works in conjunction with:

• infra(Monitoring): Expose /metrics to new port flagsmith#6533
• https://github.com/Flagsmith/pulumi/pull/156

Contributes to Flagsmith/pulumi#147

Review effort: 2/5

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.90%. Comparing base (6cb67b6) to head (85ef049).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #158      +/-   ##
==========================================
+ Coverage   95.76%   95.90%   +0.14%     
==========================================
  Files          86       90       +4     
  Lines        2949     3051     +102     
==========================================
+ Hits         2824     2926     +102     
  Misses        125      125              

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[khvn26]

Happy to approve once the coverage is sorted.