feat: add compile-time enum dimension labels via MetricDimension trait

[brayniac]

Summary

• Adds MetricDimension trait mapping enum variants to dense array indices and label strings, with a cartesian-product (A, B) tuple impl for multi-dimensional metrics
• Adds DimensionedCounter<D>, DimensionedGauge<D>, and DimensionedHistogram<D> wrappers that use D: MetricDimension to provide named accessor methods over the underlying CounterGroup/GaugeGroup/HistogramGroup
• Adds #[derive(MetricDimension)] proc macro for enums so callers avoid hand-writing the trait impl

Test Plan

• cargo nextest run --all-targets --all-features --locked — 84 tests pass
• cargo clippy --all-targets --all-features — clean
• cargo fmt --all -- --check — clean
• Integration tests in metriken/tests/dimensioned_metrics.rs cover single-dimension and cartesian-product usage, inc. .value(), .set(), .increment(), .add(), and metadata propagation

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