[exporter/splunk_hec] Max content length should apply to compressed content, not uncompressed content.

[atoulme]

Describe the bug
The Splunk HEC exporter uses the unzipped content to check the size of the payload to send, instead of the compressed content.
The max content length configuration should apply to the zipped content when compression is enabled.

Steps to reproduce
Run the exporter with compression enabled and a default max content length

What did you expect to see?
Messages sent to HEC endpoint match the max content length setting.

What did you see instead?
Messages are much smaller than the max content length enforced, as the content is compressed.

What version did you use?
Version: latest main, 0.59

Additional context
This applies for all content exported by the exporter, logs, metrics, and traces.

[github-actions]

Pinging code owners: @atoulme @dmitryax. See Adding Labels via Comments if you do not have permissions to add labels yourself.

[github-actions]

This issue has been inactive for 60 days. It will be closed in 60 days if there is no activity. To ping code owners by adding a component label, see Adding Labels via Comments, or if you are unsure of which component this issue relates to, please ping @open-telemetry/collector-contrib-triagers. If this issue is still relevant, please ping the code owners or leave a comment explaining why it is still relevant. Otherwise, please close it.

Pinging code owners:

• exporter/splunkhec: @atoulme @dmitryax

See Adding Labels via Comments if you do not have permissions to add labels yourself.