kona: chain_id metric label

Add a `chain_id` label dimension to kona's existing metric names — label only, no metric renames, no dashboard updates needed (an added label doesn't break existing queries). Update the one unit test asserting a rendered metric string.

No dependencies — parallel to everything.

🤖 *Co-created with Claude (Fable 5)*

Start by locating kona's existing metric definitions and the one unit test that asserts a rendered metric string. Add the chain_id label dimension without renaming metrics or changing dashboards, then run that unit test and confirm the rendered metric includes the label.

Written by the indexing model from the issue text.