Add scoped OpenTelemetry baggage context

[vrslev]

Summary

• Expose a shared opentelemetry_baggage_scope context manager.
• Override or remove authoritative baggage values while preserving unspecified baggage.
• Restore the parent OpenTelemetry context after success or failure.
• Keep transport propagation delegated to the existing HTTP and aio-pika instrumentation.

Verification

• Targeted OpenTelemetry and FastStream suites: 26 passed.
• Ruff format/check passed.
• MyPy passed for the changed library files.
• git diff --check passed.

Scope

This follow-up does not change FastStream middleware or Rabbit correlation IDs. The business conversation_id remains distinct from FastStream’s per-message messaging.message.conversation_id.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
unittests	98.78% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
microbootstrap/__init__.py	100.00% <ø> (ø)
...obootstrap/instruments/opentelemetry_instrument.py	98.59% <100.00%> (+0.10%)	⬆️

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