Understanding the 'metadata' Property in Annotation Payloads

The https://docs.arize.com/phoenix/tracing/how-to-tracing/capture-feedback page mentions sending the following annotation payload to attach feedback. This works!

annotation_payload = {
    "data": [
        {
            "span_id": span_id,
            "name": "user feedback",
            "annotator_kind": "HUMAN",
            "result": {"label": "thumbs-up", "score": 1},
            "metadata": {},
        }
    ]
}

However, I cannot figure out what the "metadata" property does... I don't see any changes in the UI when I send an object with random key, value pairs as metadata object. And if it should contain specific keys and values, where can I find those?