Edx-event-bus-kafka

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Changed
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* **BREAKING CHANGE**: Make event_metadata parameter required

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Changed
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* Set CloudEvent headers on events using an optional event_metadata parameter

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Changed
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* Implement openedx-events Event Bus Producer API
* **BREAKING CHANGE**: Remove caching from ``get_producer`` and rename to ``create_producer``, as we now rely on the wrapper in openedx-events to cache that call

Upgrading library from 1.x:

- Replace calls to ``edx_event_bus_kafka.get_producer`` with ``openedx_events.event_bus.get_producer``
- Add Django setting ``EVENT_BUS_PRODUCER = "edx_event_bus_kafka.create_producer"``

These breaking changes are only relevant for the producing side. (This should only include the CMS at the moment.)

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Changed
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* Improve receiver error log message -- mention that stack traces are elsewhere in log

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* Log and record receiver errors the same way as other errors (with offset, partition, etc.)

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Changed
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* Commit consumer offset asynchronously