Add Google Cloud ML Diagnostics metrics documentation (#466)

## Summary
Adds documentation (`docs/metrics.md`) and `README.md` references for Google Cloud ML Diagnostics metrics  integration in MaxDiffusion training runs.

## Key Additions
- `docs/metrics.md`: Guide for capturing metrics using `google-cloud-mldiagnostics`.
- `README.md`: Added `Metrics` section and Table of Contents link pointing to `docs/metrics.md`.