fix: cloudevents should propagate otel by default

[xytis]

This fix introduces observability dependency, so it may not be suitable for upstream.

However, after a bit of research, I came to a conclusion that unless we provide a way to customise all wrapping handlers in functions-framework-go, there is no other way to allow users to inject traceability on CloudEvent handlers.

Same applies to other non-HTTP handlers.

This fixes #42 and the issue should be known to internal google team.

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