When long-polling finds data available during the wait period, return
immediately with throttleTimeMs=0. Only use throttle time for quota
enforcement or when hitting the max wait timeout without data.
Previously, the code was reporting the elapsed wait time as throttle time,
causing clients to receive unnecessary throttle delays (10-33ms) even when
data was available, accumulating into significant latency for continuous
fetch operations.
This aligns with Kafka protocol semantics where throttle time is for
back-pressure due to quotas, not for long-poll timing information.
chrislu
bd3f67277a