Add otel_headers to OTel logic

[Kludex]

Does anyone know why just doing export OTEL_EXPORTER_OTLP_HEADERS='Authorization=your-write-token' on the terminal, before running airflow isn't enough? Airflow ignores environment variables?

[boring-cyborg]

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[Kludex]

I see an analogous PR that didn't have any tests: #42441

Can someone help me to move this forward? Also, the question no one is answering me: https://apache-airflow.slack.com/archives/C06K9Q5G2UA/p1732208803722989 😅

[Kludex]

Since #44346 was merged, we don't need this.