title: getMetricData is not using assumed role?
body: <p>Im trying to get some cloudwatch /rda metrics with the aws-sdk</p>
  
  <p>since the user Im using don't have enough permission to access cloudwatch resources, I need to assume a role at first and it seems to be ok. </p>
  
  <pre><code>sts.assumeRole(paramsts, function(err, data) {
    if (err) console.log("Cannot assume role. Received the following error: " +err.stack);
    else { 
      AWS.config.update({
        accessKeyId: data.Credentials.AccessKeyId,
        secretAccessKey: data.Credentials.SecretAccessKey,
        sessionToken: data.Credentials.SessionToken
      }); 
    console.log(data)
  </code></pre>
  
  <p>it return me a token
  with what seems to be the proper creds</p>
  
  <pre><code>{ ResponseMetadata: { RequestId: 'XXXXXXX' },
    Credentials:
     { AccessKeyId: 'XXXXXXXX',
       SecretAccessKey: 'XXXXXXXXX',
       SessionToken:
        'mytoken',
       Expiration: sometimes soon },
    AssumedRoleUser:
     { AssumedRoleId: 'blablabla',
       Arn:
        'arn:aws:sts::numbers:assumed-role/myroles' } }
  </code></pre>
  
  <p>now when in the call back of this I use getMetricData its not returning me any data. Im always getting the same answer even if Im searching for non existing item</p>
  
  <pre><code>cw.getMetricData(params, function(err, data) {
    if (err) {
      console.log("Error", err);
  
    } else {
      console.log(data);
    }
  });
  </code></pre>
  
  <p>it always output me this:</p>
  
  <pre><code>{ ResponseMetadata: { RequestId: 'XXXXx' },
    MetricDataResults:
     [ { Id: 'freestorage',
         Label: 'FreeStorageSpace',
         Timestamps: [],
         Values: [],
         StatusCode: 'Complete',
         Messages: [] }, ] }
  </code></pre>
  
  <p>it seems that the role is not properly assumed when Im using getMetricData. Am i missing something?</p>
  
  <p>Im able to fectch the data from the cli when assuming the same role  with the same parameters but this piece of code always return me 0 values/datapoint.</p>