Decoding JSON maybe undefined properties

decoding_json.re

type metricCount = {type_: string};

type metricWithLabel = {
  type_: string,
  label: string
};

let createCountWithLabel = (label: string) : metricWithLabel => {type_: "COUNT", label};

let countMetric: metricCount = {type_: "COUNT"};


let transformToCount20 = (oldJson) => {
  let label = Json.Decode.(oldJson |> optional(field("label", string)));
  // to be implemented
};