[8.18](backport #42797) [fbreceiver] - Fix batcher's configuration

[mergify]

Proposed commit message

As per batcher's documentation, there are two configurations related to flushing:

• min_size_items
  • Minimum number of log records/metric points in a request to immediately trigger a batcher flush.
  • For eg. if a plog.Log has more records than min_size_items then it is immediately forwarded to the exporter.
• max_size_items
  • Maximum log records/metric points per request.
  • For eg. if a plog.Log has more records than max_size_items then it is split into multiple requests.
  • This doesn't necessarily mean that it will be sent to exporter. That decision will be based on flush timeout or min_size_items.

This PR sets min_size_items to 0. This ensures both of the following conditions are satisfied:

1. No plog.Log contains more than bulk_max_size items.
2. Batcher flushes the data as soon it gets it

   ---

   This is an automatic backport of pull request [fbreceiver] - Fix batcher's configuration #42797 done by Mergify.

[mergify]

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Changes to be committed:
	modified:   libbeat/otelbeat/oteltranslate/outputs/elasticsearch/config_otel.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   libbeat/otelbeat/beatconverter/beatconverter_test.go
	both modified:   libbeat/otelbeat/oteltranslate/outputs/elasticsearch/config_otel_test.go

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