fix(mezmo_datadog_agent_parser): rework from initial

[biblicalph]

1. Avoid creating a full clone of the input event just for error recovery, instead passing back the (already-owned) event.
2. Fix use of .unwrap() for the log schema message field. In practice this is always set anyway.
3. Avoid wrapping and truncation for u32 casting.

Ref: LOG-22997

Summary

Vector configuration

How did you test this PR?

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

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• No

Does this PR include user facing changes?

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• No. A maintainer will apply the no-changelog label to this PR.

References

Notes

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[logdnabot]

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