perf(kura): Drop HTTP method request metric label

No linked issue.

This is a follow-up to #11025 after the Grafana cardinality dashboard still showed method multiplying kura_http_requests_total series. The route-level request counter now keeps only the bounded route label and status; request method remains available on OpenTelemetry spans where it does not multiply Prometheus series.

The Kura observability docs were updated to describe the new metric shape.

How to test locally

• mise exec -- cargo fmt
• mise exec -- cargo test render_includes_recorded_metrics
• mise exec -- cargo clippy --locked --all-targets -- -D warnings
• git diff --check