fix(derive): gate metrics variables behind feature flag

[aso20455]

Variables used only for metrics recording were causing unused variable warnings when building without the metrics feature. Wrapped these in #[cfg(feature = "metrics")] blocks to match the pattern used elsewhere in the codebase.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.1%. Comparing base (fe6dfcf) to head (69eca83).
⚠️ Report is 15 commits behind head on main.
✅ All tests successful. No failed tests found.

❗ There is a different number of reports uploaded between BASE (fe6dfcf) and HEAD (69eca83). Click for more details.

HEAD has 23 uploads less than BASE

Flag	BASE (fe6dfcf)	HEAD (69eca83)
proof	11	0
e2e	11	0
unit	2	1

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[github-actions]

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