fix(contrib/confluentinc/confluent-kafka-go/kafka.v2): ensure kafka.produce span is always finished

[pedroreys]

What does this PR do?

When a nil deliveryChan is passed to the producer.Produce(msg, nil) call, the producer wraps the producer.Events() channel and passes this wrapper channel to the inner .Produce call.

Motivation

When a nil deliveryChan was passed to the producer.Produce call, i.e. when the client was using the producer.Events() channel to get delivery reports, the kafka.produce span was never finished.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• New code is free of linting errors. You can check this by running ./scripts/lint.sh locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

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[darccio]

LGTM

[pedroreys]

@darccio thanks! Not sure what's up with the pipeline failure, tho.