Publish context

[rhhammond]

ThreeDotsLabs/watermill#445

Motivation / Background

I would like to instrument watermill-sql interactions with the database using open telemetry. Automatic instrumentation requires propagating context from callers to the database client.

Details

Message.SetContext() can be used to propagate context to the framework, and defaults to context.Background() it has not been set.

Alternative approaches considered (if applicable)

I considered updating the signature of Publish to take a context parameter, but that would be a breaking change to watermill and all supported vendors.

Checklist

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• I wrote tests for the changes.
• All tests are passing.
  • If you are testing a Pub/Sub, you can start Docker with make up.
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  • If you want to run all tests, use make test.
• Code has no breaking changes.
• (If applicable) documentation on watermill.io is updated.
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