🌱 Deduplicate metrics service lookup and port-forward in e2e steps (#2710)

SendMetricsRequest and withMetricsPortForward duplicated service
endpoint discovery and port-forward lifecycle code. Extract shared
helpers (getMetricsService, metricsPort, portForward) and rewrite
both callers to use them. Port-forward cleanup is deferred to avoid
process leaks if waitFor panics.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>