Author and validate a metric spec for the data-path suite (tests/datapath/metrics/<class>/<name>.test.yaml fixture + test_<name>.py module, run against a compose test-stand instance). Use when asked to write/scaffold/validate a test for a metric, seed bronze data for a test, add a fixture for a dashboard metric, or check a spec. Covers schemas/, templates/, $ref+sibling composition, bronze records with duplicates, account bindings, POST /v1/metric-results, and the assertion helpers (row/equals/contains, one/some over a view, approx).