fix(celery): ensure error.message tag does not include stacktrace [backport 2.9]

[github-actions[bot]]

Backport 16f3f951addfad0ba3b65235e601a9c101f273c6 from #9585 to 2.9.

Motivation

User reported that the celery exception traceback was being included in the error.message span tag. The error was due to us using the whole exception class, which includes the traceback, to set as the error message, instead of just the exception message. Updated test to ensure just error message is returned.

Checklist

• [x] Change(s) are motivated and described in the PR description
• [x] Testing strategy is described if automated tests are not included in the PR
• [x] Risks are described (performance impact, potential for breakage, maintainability)
• [x] Change is maintainable (easy to change, telemetry, documentation)
• [x] Library release note guidelines are followed or label changelog/no-changelog is set
• [x] Documentation is included (in-code, generated user docs, public corp docs)
• [x] Backport labels are set (if applicable)
• [x] If this PR changes the public interface, I've notified @DataDog/apm-tees.

Reviewer Checklist

• [x] Title is accurate
• [x] All changes are related to the pull request's stated goal
• [x] Description motivates each change
• [x] Avoids breaking API changes
• [x] Testing strategy adequately addresses listed risks
• [x] Change is maintainable (easy to change, telemetry, documentation)
• [x] Release note makes sense to a user of the library
• [x] Author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• [x] Backport labels are set in a manner that is consistent with the release branch maintenance policy

[datadog-dd-trace-py-rkomorn[bot]]

Datadog Report

Branch report: backport-9585-to-2.9 Commit report: 2ae2cd1 Test service: dd-trace-py

:white_check_mark: 0 Failed, 226 Passed, 50 Skipped, 19m 18.67s Total duration (1.19s time saved)

[pr-commenter[bot]]

Benchmarks

Benchmark execution time: 2024-07-01 12:18:51

Comparing candidate commit 2ae2cd1312b891dc7e5c5cb40c4a65acb32bd58d in PR branch backport-9585-to-2.9 with baseline commit a3010a9f20ffd2bc8734599b1b54a99eb9703f0c in branch 2.9.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 209 metrics, 9 unstable metrics.