Create shared baseline_common/metrics.rs module that registers all 18
dashboard-expected metrics (11 gauges, 5 counters, 2 histograms) and
spawns a lightweight HTTP metrics server on port 9094.
Instrument train_baseline_supervised and train_baseline_rl with:
- Epoch progress, training/validation loss gauges
- Checkpoint save timing, size, and failure counters
- NaN/gradient explosion detection counters
- Data loading latency histograms
- Active workers lifecycle (1 on start, 0 on exit)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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