aws: add debug logs to check batch sizes

[PettitWesley]

A common complaint from users is how to determine whether Fluent Bit is efficiently batching records to the AWS APIs. This can be altered with the Flush setting. But users need a way to check the batch size. This adds debug logs that users can easily search for that print the batch size in # of events and # of bytes.

AWS team will add this to our debugging guide once this commit is released.

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Testing
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Backporting

• Backport to latest stable release.

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.