test: cover Metric constructor validation

[WilliamK112]

Summary

• add direct tests for the shared Metric constructor validation contract
• verify the valid path applies defaults, sorts labels, and calls reset()
• include the required changelog entry

This is test-only and does not change runtime behavior.

Refs #805 (focused coverage improvement; intentionally does not close the broader issue)

Coverage

For lib/metric.js in the full suite:

• statements/lines: 82.14% → 96.42%
• branches: 78.26% → 95.65%

Testing

• npm test — lint, Prettier, TypeScript, 30 suites and 588 tests pass
• focused test on Node.js 22 — 5 tests pass
• focused test on Node.js 24 — 5 tests pass
• full suite and coverage on Node.js 26 — 588 tests pass