Added easy to interpret metrics for observability and related tests via Prometheus

[shantanushok]

Description

Resolves #919

Added easy to interpret metrics for observability and related tests via Prometheus. This ensures that CSRF evaluations (allowances, rejections, and safe method skips) are correctly tracked and monitored.

Summary of Changes

• backend/middleware/csrf.py:

  • Imported Counter from prometheus_client and defined CSRF_REQUESTS and CSRF_REJECTIONS metrics.
  • Updated OriginValidationMiddleware to increment csrf_requests_total for safe methods (skipped_safe_method), authorized origins (allowed), and unauthorized cross-origin requests (rejected).
  • Added tracking for csrf_rejections_total to specify the HTTP method and invalid_origin reason when a request is blocked.

• backend/tests/test_csrf.py:

  • Fixed the failing test_prometheus_metrics regression test, which was previously failing due to AssertionError: assert 0.0 > 0.0 caused by the missing middleware implementation.

Verification

Ran backend CSRF validation and metrics regression tests:

python -m pytest backend/tests/test_csrf.py -v

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Pull request overview

This PR adds Prometheus counters to the CSRF OriginValidationMiddleware so CSRF decision outcomes (safe-method skip, allowed, rejected) and rejection reasons can be tracked for observability, and it extends the CSRF test suite with a regression test that asserts these metrics increase as expected.

Changes:

• Added Prometheus Counter metrics in backend/middleware/csrf.py and incremented them on safe-method skips, allows, and rejects (including method + reason for rejects).
• Added a CSRF Prometheus metrics regression test in backend/tests/test_csrf.py to validate counter increments via real requests through the middleware.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
backend/middleware/csrf.py	Introduces csrf_requests_total and csrf_rejections_total counters and increments them on CSRF decision paths.
backend/tests/test_csrf.py	Adds a metrics-focused regression test that checks Prometheus counters increase after representative requests.

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[imDarshanGK]

@shantanushok

[shantanushok]

Hey @imDarshanGK please review the PR and hopefully merge it :/