contrib/confluentinc/confluent-kafka-go: fix goroutine leak in Produce

[rarguelloF]

What does this PR do?

(Thanks to @majorgreys who reported this issue).

Fix a goroutine leak in producer.Produce when the actual Produce call was returning an error and a deliveryChan is provided.
In this case, the goroutine "wrapping" the original channel gets stuck waiting for a message in the channel, but no event is ever sent to that channel.

Motivation

Fix goroutine leak.

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.
• 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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[pr-commenter]

Benchmarks

Benchmark execution time: 2024-10-11 15:53:48

Comparing candidate commit 8899d7f in PR branch rarguelloF/kafka-goroutine-leak with baseline commit 5b9a8af in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 58 metrics, 1 unstable metrics.