code: function makeKey(metricName, dimensions) {
    let key = metricName;
    for (const {Name, Value} of dimensions) {
      key += `\0${Name}\0${Value}`;
    }
    return key;
  }
docs: Make a key for a given metric name and some dimensions. This works on the
  assumption that sane people won't put a null character in their metric name
  or dimension name/value.
  
  @param {String} metricName
  @param {Object[]} dimensions
  @returns {String} Something we can actually use as a Map key.