Remove setting of headers in OTLPSpanExporter and OTLPMetricExporter in Airflow 2.11.0

[doyong365]

This PR fixes an issue in the Airflow 2.11.0 stable branch where headers were hardcoded in OTLPSpanExporter and OTLPMetricExporter, preventing users from properly configuring authentication via custom headers.

By removing the hardcoded headers, this change enables users to add necessary authentication information when exporting spans using OTLPSpanExporter.

This issue is specific to Airflow 2.x; while Airflow 3.0.x have breaking changes, many users still rely on Airflow 2.11.0 for production environments and require this bugfix.

related: PR#44346

[boring-cyborg]

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[potiuk]

We only plan to release absolutely crirical issues in 2.11. That does not seem like one. You can continue patching your version of course, and ugprade to Airflow 3 at any convenience.