fix(ratelimiter): log metrics server bind failures at error level

[rootkiller6788]

What

setupMetrics in src/invocation-plane-services/ratelimiter/cmd/main.go discarded the http.ListenAndServe error in an empty branch:

err := http.ListenAndServe("0.0.0.0:7776", mux)
if err != nil {

}

If the Prometheus listener cannot bind, the service keeps running with no metrics and logs nothing — scrapes fail with connection refused and there is no signal in the pod logs. This fixes #540.

Change

• Log the bind/serve failure at error level, matching the sibling setupPprof and setupOlricStats servers in the same file.
• Add metrics_test.go: a regression test that occupies 0.0.0.0:7776 so the bind fails deterministically, swaps the global logger for a zaptest/observer, and asserts the metrics server failed log line is emitted.
• Wire metrics_test.go into the cmd_test bazel target.

Verification

• go build ./cmd/... passes.
• go vet ./cmd/ passes.
• go test ./cmd/ -run TestSetupMetricsLogsServeFailure -count=1 passes.

The empty branch is the only code path the fix touches; the error log uses the same zap.L().Error(...) idiom already used by the other two servers in the file.

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