out_prometheus_exporter: Return 204 on no metrics

[jplitza]

Especially when using log_to_metrics or directly after startup, it might well be that no metrics are available yet. This isn't an error condition, so don't return a 4xx code!

Fixes #9366

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[jplitza]

@edsiper Thanks for triggering the Actions! The failing test about in_kubernetes on macos and the failing build on s390x don't seem to be related to my change.