Eval metrics now delegate to rankops::metrics instead of fynch::metrics.

Also repairs the serde feature under --all-features: aefff75 removed the
HashMap import as unused (true on default features only; export_to_json
is serde-gated), which left the serde build broken. The import is now
feature-gated, and the serde-gated Metrics struct fields get docs to
satisfy missing_docs. CI runs default features only, so the break was
invisible locally and in CI; caught during pre-publish all-features
verification.