In order to display multi-line text metadata you will have to use special characters "\r\n" within the value of the iMetadata object. The following is a sample JSON of the insert request:

{

    "imetadata": {

        "key": "<metadataKeyHere>",

        "value": "First row,\r\nSecond row"

    },

    "target": {

        "id": "<xcontentIdHere>",

        "entityType": "CONTENT"

    }

}