out_datadog: Fix formatting issue when setting a static hostname via dd_hostname (backport 3.2)

[ddrthall]

Providing a static hostname via the dd_hostname configuration options is currently bugged, with an off-by-one error in the message packing loop causing dropped attributes and/or malformed output json. This PR corrects the issue, incrementing the msgpack map size by one when a dd_hostname property is detected.

This PR is a backport of !10104

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Testing
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Backporting

• Backport to latest stable release.

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[github-actions]

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