fix: OtlpHttpLogRecordExporter request headers

[Waheiba]

This addresses a similar issue as #877
Headers values are added twice which concatenates the same value again over the existing for the same header.

createRequest(body:endpoint:) in OtlpHttpExporterBase already considers envVarHeaders and config.headers
Additional looping on those headers in OtlpHttpTraceExporter after the request is created is appending the same value again.

Tests are also added

Apologies for the other PR that I closed. Small accident with commit history.

[bryce-b]

Apologies for the other PR that I closed. Small accident with commit history.

I hate it when that happens :)

Thanks for the contribution @Waheiba
Could you extend this change to the other exporters as well?
Nevermind, I thought this issue was with all the exporters

[Waheiba]

Thanks for the contribution @Waheiba

My pleasure :)

I couldn't find in the docs how to request or start a release. If you could point this out would be great. Creating a pre-release version would be awesome after merging this PR.