Add end-to-end test for metrics API functionality

- Tests node metrics retrieval from cluster
- Tests pod metrics retrieval with namespace filtering
- Tests pod metrics with label selector filtering
- Tests multi-namespace metrics aggregation
- Includes proper error handling and skip logic if metrics-server unavailable
- Creates test pod, waits for it to be running, then fetches metrics

Co-authored-by: brendandburns <5751682+brendandburns@users.noreply.github.com>