aws-sdk for node.js でWindowsの空きメモリ量をCloudWatchにプロットする

var AWS = require('aws-sdk');
var os = require('os');

new AWS.MetadataService().request("/latest/meta-data/instance-id", function (err, data) {
  var instanceId = data;
  var total = os.totalmem();
  var free = os.freemem();    
  var value = (total - free) * 100 / total;
  var cloudwatch = new AWS.CloudWatch({region: 'ap-northeast-1'});
  var params = {
    MetricData: [
      {
        MetricName: 'FreeMemorySpace',
        Dimensions: [
          {
            Name: 'InstanceId', 
            Value: instanceId 
          },
        ],
        Timestamp: new Date (),
        Unit: 'Megabytes',
        Value: free / 1048576.0
      },
    ],
    Namespace: 'Windows System' /* required */
  };
  cloudwatch.putMetricData(params, function(err, data) {
    if (err) console.log(err, err.stack); // an error occurred
    else     console.log(data);           // successful response
  });
});