Move readProcStat, getCpuPercent, getDisk and the cpu/memory/disk payload
assembly out of index.js into a standalone metrics.js, so the upcoming
local workstation agent can reuse the same collection code. No behaviour
change: /health returns the exact same fields, in the same order.
Notes:
- collectMetrics() returns the { cpu, memory, disk } slice; getHealth()
spreads it, keeping the JSON key order the admin dashboard relies on.
- getHealth() keeps Promise.all([collectMetrics(), getLogtoHealth()]).
The 500ms CPU sample and the 3s Logto check are deliberately concurrent;
serializing them would push the p99 of /health to ~3.5s.
- collectMetrics() awaits the CPU sample as its only await, so callers
running it inside a Promise.all keep their concurrency.
- Dockerfile COPY lists files one by one, so metrics.js had to be added
there or the container would crash on MODULE_NOT_FOUND at startup.
- New __tests__/health.test.js: metrics.js exports, field-for-field
payload shape, and a latency guard (<1.5s with a 1200ms stubbed Logto).
Verified by mutation: serializing the two calls fails the latency test
at ~1743ms while the field comparison still passes.
- Runtime stays 0-dependency; the 14 existing tests are untouched.
Resolves#11
