logstash gc cost much time and too many 429.

when the fluent-bit send lots of log.

the logstash would return many 429.

i think it is the gc problem.

#### logstash version
logstash 8.15, jdk 21, g1 gc

-Xms Xmx 6G

memory limit 12GB.

No source files, tests, or entry points are named. Start by reproducing the issue with Fluent Bit sending a high log volume to Logstash 8.15 using JDK 21, G1 GC, and the stated memory limits; correlate GC pauses with 429 responses. Done means the cause of the 429 responses is established and the behavior is validated under the reported workload.

Written by the indexing model from the issue text.