contrib/slog: clone record before calling Add

[felixge]

What does this PR do?

Fix a bug that could manifest itself when a slog Record is modified concurrently (A) with our handler.Handle implementation (B). If A completes before B, then it might overwrite an attribute added by A.

Add test case demonstrating the issue.

Fixes #2918

Motivation

#2918

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: 2024-10-15 12:00:43

Comparing candidate commit 7362172 in PR branch gh-2918-slog-add-clone with baseline commit 93311db in branch main.

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