Add new flag --enable_rich_metrics

[ycchenzheng]

Description

Add a new flag --enable_rich_metrics and output a JSON file that has the following data which is more comprehensive:

{
  "avg_step_time": "20s",
  "min_step_time": "19s on step 15",  # This can be split for better parsing
  "max_step_time": "25s on step 35",
  "goodput": "85%",
  "MFU": "100 Tflops/s"
  "badput_breakdown": {
    "checkpoint_loading": "2%",
    "data_loading": "5%",
    "failure": "5%",
    "reshard": "0.5%"
  },
  "links": {
    "cloud_logging": "...",
    "goodput_monitor": "...",
    "disruption_dashboard": "..."
  }
}

This will make it far easier to actually parse through the major landmarks needed in tests

FIXES: b/409799213

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Tests

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Checklist

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• I have performed a self-review of my code.
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• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed.