feat(sdk): enrich collected metrics — granular status codes, TTFB, request size, inflight

[Fabien83560]

feat(sdk): enrich collected metrics — granular status codes, TTFB, request size, inflight

Context

The current schema aggregated HTTP status codes into three fixed buckets (2xx/4xx/5xx),
making it impossible to distinguish a 401 from a 429 or a 404 at the insight level.
This PR extends the data collection layer with higher-value signals while keeping the
privacy-first guarantee intact — no body content, no PII, sizes only.

Also fixes a silent bug: lat_avg was computed in the aggregator but never written to SQLite.

Changes

• status_3xx — redirections tracked as a dedicated counter
• status_dist — per-code granular distribution stored as JSON ({"200":45,"404":3})
• lat_ttfb_p50/p90/p99 — Time To First Byte captured via res.write/res.end patch
• request_size_avg — request body size from req.headers['content-length']
• inflight_avg / inflight_max — concurrent request count snapshot at processing time
• lat_avg — bug fix: was computed but missing from INSERT statement and schema
• Automatic migrations for all new columns (silent ALTER TABLE on existing databases)
• Version bumped 2.1.2 → 2.2.0

How to test

npm install
npm run tests   # 82/82 pass

Manual validation: add the middleware to any Express app, make a few requests with
different status codes, then query .apiforge.db:

SELECT route, status_dist, lat_ttfb_p90, inflight_max FROM api_metrics LIMIT 5;

Checklist

• Tests added / updated (82 pass, 20 new tests)
• Documentation updated
• No breaking change (new nullable columns, backward-compatible migrations)
• Reviewed locally