Automatic merge from submit-queue (batch tested with PRs 52316, 52289, 52375)

[fluentd-gcp addon] Trim too long log entries due to Stackdriver limitations

Stackdriver doesn't support log entries bigger than 100KB, so by default fluentd plugin just drops such entries. To avoid that and increase the visibility of this problem it's suggested to trim long lines instead.

/cc @igorpeshansky

```release-note
[fluentd-gcp addon] Fluentd will trim lines exceeding 100KB instead of dropping them.
```