Commit 73eca4f6a wrapped /metrics, /metrics/aggregated and /system/metrics
behind a new MetricsProtection middleware. Without auth they return 403,
which broke the 6 metrics sub-tests. The middleware reads
METRICS_BEARER_TOKEN at construction time, so set it via t.Setenv before
calling setupTestRouter, and add a needsMetricsAuth flag on the test
case so the request carries the matching Authorization header.
15b29f6620