Automatic merge from submit-queue

Update fluentd-gcp configuration

Related to #32762

Though it's not a final solution to the fluentd OOM problems, it increases number of logs that can be handled without losses by
- switching to the file buffering, making buffering mechanism more resilient
- decreasing size of the buffer, decreasing the amount of memory needed
- decreasing number of threads handling the load, since number of chunks is lower than previous number of threads

which results in decrease in theoretical throughput. Tests to confirm cases covered by this change will follow.

cc @piosz @edsiper @repeatedly please take look and confirm that all of these changed are meaningful.