feat: F09 Request Metrics — complete deferred items

[leocamello]

Summary

Complete all 23 deferred Phase 7 items for F09 Request Metrics. Every task is now checked [x] (78/78).

Added

• 22 integration tests (tests/metrics_integration.rs): Contract tests for /metrics and /v1/stats, counter/histogram/gauge tracking, per-backend stats breakdown, content-type validation, comprehensive end-to-end test, FR-020 compatibility
• 4 Criterion benchmarks (benches/metrics.rs): metric recording overhead (~188ns), /metrics render (~3.8µs), /v1/stats compute (~5.9µs), label sanitization (~50ns cached / ~359ns uncached)
• 2 property tests (src/metrics/mod.rs): Prometheus label regex compliance, sanitization idempotency
• requirements-validation.md: Spec quality gate checklist (56 checked, 9 N/A, 0 unchecked)
• README Observability section: Documents /metrics and /v1/stats endpoints

Changed

• /metrics endpoint now returns proper Prometheus content-type: text/plain; version=0.0.4 header (T068)
• quickstart.md: Fixed dependency versions (0.24/0.16), module structure (no collector.rs), handler code, startup initialization, and test examples to match actual implementation (T073)
• Cargo.toml: Added [[bench]] entries with harness = false for all Criterion benchmark files (cli_startup, config_parsing, metrics) — benchmarks were previously compiling but not executing

Updated

• tasks.md: 78/78 items complete (0 deferred, 0 unchecked)
• verification.md: VER-003 updated to reflect fully completed deferred work

Test Results

• 389 tests pass (284 unit + 105 integration)
• Clippy clean (-D warnings)
• All benchmarks pass with overhead well under 0.1ms requirement
• Format check clean

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.38%. Comparing base (ebbbbde) to head (41c0bd0).
✅ All tests successful. No failed tests found.
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #108      +/-   ##
==========================================
+ Coverage   78.45%   82.38%   +3.92%     
==========================================
  Files          34       34              
  Lines        1453     1453              
==========================================
+ Hits         1140     1197      +57     
+ Misses        313      256      -57     

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