feat: add /metrics endpoint and middleware for usage statistics

[SenseiSuraj24]

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Suggested PR Title: feat: add /metrics endpoint and middleware for usage statistics

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Description

This PR introduces a new /metrics endpoint that returns real-time usage statistics about the running server, using in-memory counters as requested.

Specifically, this PR:

1. Adds a global METRICS dictionary in backend/app/main.py to store total_requests, total_analyses, and a dictionary for languages_detected.
2. Implements a new HTTP middleware (metrics_middleware) that safely increments total_requests for every incoming request and parses successful POST responses to analysis endpoints (/explanation/, /debugging/, /suggestions/, /analyze/) to increment total_analyses and the specific languages_detected.
3. Adds the GET /metrics route returning the properly formatted JSON payload including calculated uptime_seconds and the current API version.
4. Adds the test_metrics_endpoint function in backend/tests/test_endpoints.py to ensure the metrics accurately reflect the server's traffic and state.

Related Issue

Fixes #

Type of change

• Bug fix
• New feature / enhancement
• Documentation update
• Test addition
• Refactor

Checklist

• I have read CONTRIBUTING.md
• My branch is up to date with main
• I have run pytest -v and all tests pass
• I have not introduced duplicate issues or features
• My PR title follows the format: feat/fix/docs/test: short description
• I have added tests for new features (Level 2 and 3 issues)
• No hardcoded secrets or API keys in my code
• This PR is linked to a GSSoC 2026 issue

Screenshots (if frontend change)

N/A — Backend only changes.

Test evidence

pytest -v
============================= test session starts ==============================
platform win32 -- Python 3.12.0, pytest-8.0.0, pluggy-1.4.0 -- 
cachedir: .pytest_cache
rootdir: C:\Users\SURAJ\Desktop\gssoc5\AI-dev-assistant\backend
collected 40 items

tests/test_endpoints.py::test_root PASSED                                [  2%]
tests/test_endpoints.py::test_health PASSED                              [  5%]
tests/test_endpoints.py::test_rate_limit_headers_on_success_response PASSED [  7%]
tests/test_endpoints.py::test_rate_limit_returns_429_with_retry_after_header PASSED [ 10%]
tests/test_endpoints.py::test_explanation_python PASSED                  [ 12%]
tests/test_endpoints.py::test_explanation_no_language_hint PASSED        [ 15%]
tests/test_endpoints.py::test_explanation_rust PASSED                    [ 17%]
tests/test_endpoints.py::test_explanation_detects_rust_without_hint PASSED [ 20%]
tests/test_endpoints.py::test_explanation_accepts_rust_hint_alias PASSED [ 22%]
tests/test_endpoints.py::test_explanation_empty_code PASSED              [ 25%]
tests/test_endpoints.py::test_explanation_too_long PASSED                [ 27%]
tests/test_endpoints.py::test_explanation_typescript PASSED              [ 30%]
tests/test_endpoints.py::test_explanation_java PASSED                    [ 32%]
tests/test_endpoints.py::test_explanation_cpp PASSED                     [ 35%]
tests/test_endpoints.py::test_explanation_cyclomatic_fields_present PASSED [ 37%]
tests/test_endpoints.py::test_explanation_cyclomatic_simple PASSED       [ 40%]
tests/test_endpoints.py::test_explanation_cyclomatic_moderate PASSED     [ 42%]
tests/test_endpoints.py::test_explanation_cyclomatic_high PASSED         [ 45%]
tests/test_endpoints.py::test_explanation_cyclomatic_very_high PASSED    [ 47%]
tests/test_endpoints.py::test_debug_detects_zero_division PASSED         [ 50%]
tests/test_endpoints.py::test_debug_detects_hardcoded_secret PASSED      [ 52%]
tests/test_endpoints.py::test_debug_detects_bare_except PASSED           [ 55%]
tests/test_endpoints.py::test_debug_detects_eval PASSED                  [ 57%]
tests/test_endpoints.py::test_debug_clean_code PASSED                    [ 60%]
tests/test_endpoints.py::test_debug_javascript PASSED                    [ 62%]
tests/test_endpoints.py::test_debug_java PASSED                          [ 65%]
tests/test_endpoints.py::test_debug_cpp PASSED                           [ 67%]
tests/test_endpoints.py::test_explanation_php PASSED                     [ 70%]
tests/test_endpoints.py::test_explanation_detects_php_without_hint PASSED [ 72%]
tests/test_endpoints.py::test_debug_php PASSED                           [ 75%]
tests/test_endpoints.py::test_debug_php_buggy_patterns PASSED            [ 77%]
tests/test_endpoints.py::test_debug_rust PASSED                          [ 80%]
tests/test_endpoints.py::test_debug_rust_buggy_patterns PASSED           [ 82%]
tests/test_endpoints.py::test_debug_kotlin PASSED                        [ 85%]
tests/test_endpoints.py::test_debug_cpp_syntax_errors PASSED             [ 87%]
tests/test_endpoints.py::test_debug_issue_has_required_fields PASSED     [ 90%]
tests/test_endpoints.py::test_js_ts_security_patterns PASSED             [ 92%]
tests/test_endpoints.py::test_suggestions_returns_score PASSED           [ 95%]
tests/test_endpoints.py::test_suggestions_perfect_score PASSED           [ 97%]
tests/test_endpoints.py::test_metrics_endpoint PASSED                    [100%]

============================= 40 passed in 1.42s ===============================

[SenseiSuraj24]

Hey @imDarshanGK I have completely resolved all the issues and have also implemented the required functions and checked through all the tests , i have updated the info in the same way , you can go ahead for the merge ,
Please do gssoc:approved tags and level , type tags to so i can receive my GSSoc ' 26 points accordingly
Thank you

[Copilot]

Pull request overview

This PR adds basic in-memory usage metrics to the FastAPI backend by introducing a /metrics endpoint and a middleware that counts requests and analyses, plus a test validating the metrics payload.

Changes:

• Added global in-memory metrics state (request count, analysis count, languages histogram) and server start timestamp.
• Introduced an HTTP middleware that increments counters and attempts to infer language from successful analysis responses.
• Added GET /metrics endpoint and a new test_metrics_endpoint in the existing endpoint test suite.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
backend/app/main.py	Adds metrics state, metrics middleware, and /metrics endpoint implementation.
backend/tests/test_endpoints.py	Adds a new test that resets metrics, performs a few requests, and asserts on /metrics output.

Comments suppressed due to low confidence (2)

backend/app/main.py:89

• This middleware fully buffers every successful analysis response (response_body = [...] / b"".join(...)) to extract the language. That defeats streaming and adds extra allocations per request. Prefer extracting the language from the request payload (or a dedicated header/field) so you don’t need to consume and rebuild response.body_iterator.

            try:
                response_body = [chunk async for chunk in response.body_iterator]
                response.body_iterator = iterate_in_threadpool(iter(response_body))
                body_bytes = b"".join(response_body)
                data = json.loads(body_bytes)

backend/app/main.py:83

• The middleware claims to track analyses across /explanation/, /debugging/, /suggestions/, and /analyze/, but test_metrics_endpoint only exercises /explanation/ and /analyze/. Add assertions covering the /debugging/ and /suggestions/ branches (including how languages_detected should behave for those routes) to prevent regressions.

    if request.url.path in ("/explanation/", "/debugging/", "/suggestions/", "/analyze/") and request.method == "POST":
        if response.status_code == 200:
            METRICS["total_analyses"] += 1

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[SenseiSuraj24]

hey @imDarshanGK I have resolved all the conflicts which were suggested by co pilot too , please do add the needful tags and labels mainly gssoc:approved and go ahead for a merge !!
Thank you