Add L7 proxy observability

[kacy]

Summary

• add L7 proxy request, response, retry, loop-rejection, and upstream-failure counters to proxy runtime state
• expose those counters in service rollout status and Prometheus metrics
• add L7 access-style logging with per-connection trace IDs for proxied requests and failure paths

Validation

• zig fmt on touched files
• single-process targeted zig build test -Doptimize=ReleaseSafe -- --test-filter ... runs for proxy runtime and rollout status/metrics coverage
• each targeted run hit the explicit timeout without emitting failures