feat: add hostname cardinality cap override

[rickcrawford]

Summary

• Closes WOR-89 by adding proxy.metrics.cardinality.hostname_cap for the public hostname label budget.
• Wires the metrics config into the runtime cardinality limiter before request metrics are recorded.
• Unignores the hostname overflow e2e regression and documents the new config knob.

Test plan

• RED: cargo build -p sbproxy && cargo test -p sbproxy-e2e --test metrics_per_agent cardinality_overflow_emits_sentinel_and_increments_demotion_counter failed because the lowered hostname cap was not applied
• GREEN: cargo test -p sbproxy-observe hostname_cap_override_wins_over_adr_budget
• cargo test -p sbproxy-observe record_request_with_labels_stamps_agent_dimensions
• cargo test -p sbproxy-config --lib
• cargo build -p sbproxy && cargo test -p sbproxy-e2e --test metrics_per_agent cardinality_overflow_emits_sentinel_and_increments_demotion_counter
• cargo test -p sbproxy-observe --lib
• cargo fmt --all -- --check
• git diff --check
• IDE lints: no errors for edited files

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