[type:perf] kafka logging: flush producer once per batch instead of p…

[renxf0319]

What this PR do

Optimize logging‑kafka plugin performance:

• Move producer.flush() out of record loop, flush once per batch logs instead of per record.
• Avoid frequent blocking flush, restore kafka producer batch capability.
• Minor optimize: extract config map outside loop, add topic blank defensive check, adjust log level.

Background

The original code calls producer.flush() after every record send, which disables Kafka batch mechanism and seriously degrades gateway throughput.