Fix a log schema inconsistency (kubernetes/namespace)

What does this MR do?

In most of the application, structured logs that emit an "exception" key for errors are sent with a Hash in this key. In this class, a string was used. This frustrates Elasticsearch, which is commonly used to store structured logs, causing it to have to drop logs from one instance or the other due to the type mismatch.

Related: https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/8931#note_290078183

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

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