example snippet that produces the following json:
dvc metrics show --all-tags data/output/metrics_eval.yaml --show-json > all_metrics.json
export METRICS_JSON=$PWD/all_metrics.json
node .github/prep_metrics.js
echo "## Plots" >> report.md
echo "### Precision History" >> report.md
vl2png precision-vega.json | cml publish --md >> report.md