[exporter/splunkhec] Support partial resends

[atoulme]

Component(s)

exporter/splunkhec

Is your feature request related to a problem? Please describe.

The splunk_hec exporter doesn't support partial acceptance of records sent to the backend. The Splunk backend can offer information on how many events it was able to process before sending an error. A typical HTTP response body is:

{"text":"Server is busy","code":9,"invalid-event-number":10}

The invalid-event-number key indicates when the payload processing stopped, for example 10 means that it stopped on the 10th one, and we can try resending by dropping the first 9 events.

Describe the solution you'd like

Implement support to read the response, even on 503, and make sure to drop events if possible based on the feedback from the server.

Describe alternatives you've considered

No response

Additional context

No response

[github-actions]

Pinging code owners:

• exporter/splunkhec: @atoulme @dmitryax

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