feat: expand prometheus http metrics

[pq198363-ops]

Closes #38.

Summary

• add agentpay_http_requests_total{method,route,status} counters for completed HTTP responses
• add agentpay_http_request_duration_seconds histogram buckets, sums, and counts
• add agentpay_http_errors_total{type} increments from the terminal Express error handler
• keep the existing gauges and Prometheus text exposition format
• record route labels from matched Express route patterns such as /api/v1/usage/:agent/:serviceId, using unmatched for parser failures and unknown routes to avoid raw-path cardinality
• document all metrics in docs/metrics.md

RED evidence

Before implementation, the focused metrics test failed because /api/v1/metrics only exposed the existing gauges and did not include:

• agentpay_http_requests_total
• agentpay_http_request_duration_seconds
• route-pattern labels for dynamic usage reads
• agentpay_http_errors_total for terminal error-handler paths

Verification

• npx prettier --check README.md docs/metrics.md src/metrics.ts src/metrics.test.ts src/middleware/index.ts src/routes/metrics.ts src/routes/errors.ts
• npm run lint
• clean dist/, then npm run build
• $env:NODE_ENV='test'; node --test dist/*.test.js dist/**/*.test.js
• git diff --check

Full clean test result: 92 tests, 11 suites, all passed.