[exporter/kafka] Add record_headers config to set static headers on outgoing records

[singhvibhanshu]

Description

Resolves #47193

This PR adds a new record_headers configuration option to the Kafka exporter

Changes made:

• added record_headers to Config struct
• updated makeFranzMessages in the franz-go client wrapper to iterate over the configured map and append them as kgo.RecordHeader to the outgoing messages
• added TestMakeFranzMessages_RecordHeaders to verify headers are successfully attached, bringing test suite to 147 passing tests
• documented the new field and provided an example in the README.md
• added .chloggen/fix-47193.yaml

Testing

• added TestMakeFranzMessages_RecordHeaders
• all 147 tests passed locally

[axw]

The issue hasn't been discussed yet. Marking this as draft until agreement is reached.

[khushijain21]

@singhvibhanshu I think what this is missing is precedence order.

I think we would want include_metadata_key to take precedence if it exists. @paulojmdias wdyt?

[paulojmdias]

I think we would want include_metadata_key to take precedence if it exists. @paulojmdias wdyt?

Mixed feelings about that, but let's discuss in the issue to have a proper agreement there with other codeowners 👍

[axw]

Thanks @singhvibhanshu. Looks good, just a few comments.

[singhvibhanshu]

This PR got merge conflict.
I'll fix it soon.

[axw]

LGTM, minus the merge conflict. Thanks!

[singhvibhanshu]

I've resolved the merge conflicts and modfied the test.
Kindly take a look when you get a chance.
Thank you!

[paulojmdias]

LGTM

[singhvibhanshu]

/rerun

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