Note

Low Risk
Benchmark-only change that adds new counters and wires them into NetworkMetrics; low blast radius, with minimal risk beyond potential metrics registration/labeling mistakes.

Overview
The broadcast network stress test node now emits topic-scoped broadcast metrics: counters for sent/received messages and a labeled counter for dropped messages by drop reason.

create_network_metrics() now builds a BroadcastNetworkMetrics entry keyed by TOPIC.hash() and supplies it via NetworkMetrics.broadcast_metrics_by_topic (instead of leaving it None), enabling the network layer to attribute metrics to the stress-test broadcast topic.

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