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Measures the per-message Data Streams Monitoring hot path for the kafkajs producer: getMessageSize plus DsmPathwayCodec.encode, which varint-encodes the pathway context into a reused scratch buffer and base64s it into the message headers. Variants cover a small keyed message, a larger message with user headers, and a mixed batch. Header trace-context injection is covered by the propagation bench and is not duplicated here.