feat: propagate W3C baggage headers into OTel context

[betinacosta]

Disclaimer: Do not include SAP-internal or customer-specific information in this PR (e.g. internal system URLs, customer names, tenant IDs, or confidential configurations). This is a public repository.

Description

When using auto_instrument(middlewares=[StarletteIASTelemetryMiddleware(...)]), W3C baggage headers from incoming HTTP requests were not visible on spans, as baggage is not extracted from the request headers into the OTel context. This PR fixes it.

Related Issue

Closes #87

(Link to the GitHub issue this PR addresses)

Type of Change

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• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• Dependency update

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• I have added/updated automated tests to cover my changes
• All tests pass locally
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Breaking Changes

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