fix(kafka): cast topic to str since it can return None

[ZStriker19]

Since topic can be None for a message object we should always cast it to a str or else it'll throw an error if we try to set it as a tag with set_tag_str

https://docs.confluent.io/platform/current/clients/confluent-kafka-python/html/index.html#confluent_kafka.Message.topic

Checklist

• PR author has checked that all the criteria below are met
• The PR description includes an overview of the change
• The PR description articulates the motivation for the change
• The change includes tests OR the PR description describes a testing strategy
• The PR description notes risks associated with the change, if any
• Newly-added code is easy to change
• The change follows the library release note guidelines
• The change includes or references documentation updates if necessary
• Backport labels are set (if applicable)

Reviewer Checklist

• Reviewer has checked that all the criteria below are met
• Title is accurate
• All changes are related to the pull request's stated goal
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Newly-added code is easy to change
• Release note makes sense to a user of the library
• If necessary, author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[github-actions]

CODEOWNERS have been resolved as:

releasenotes/notes/kafka_message_topic_none_fix-2bc3fd6388075b94.yaml   @DataDog/apm-python
ddtrace/contrib/internal/kafka/patch.py                                 @DataDog/apm-core-python @DataDog/apm-idm-python

[datadog-dd-trace-py-rkomorn]

Datadog Report

Branch report: zachg/cast_topic_to_str_before_tagging
Commit report: 8464af9
Test service: dd-trace-py

✅ 0 Failed, 592 Passed, 604 Skipped, 19m 4.8s Total duration (17m 43.61s time saved)

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-09-20 15:38:06

Comparing candidate commit 8464af9 in PR branch zachg/cast_topic_to_str_before_tagging with baseline commit baac738 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 348 metrics, 48 unstable metrics.