title: Python: Boto3: get_metric_statistics() only accepts keyword arguments
body: <p>Just started using Boto3 with Python so definitely new at this. </p>
  
  <p>I'm trying to use a simple get_metric_statistics script to return information about CPUUtilization for an instance. Here is the script I'm looking to use:</p>
  
  <pre><code>import boto3
  import datetime
  
  cw = boto3.client('cloudwatch')
  
  cw.get_metric_statistics(       
          300,
          datetime.datetime.utcnow() - datetime.timedelta(seconds=600),
          datetime.datetime.utcnow(),
          'CPUUtilization',
          'AWS/EC2',
          'Average',
          {'InstanceId':'i-11111111111'},
          )
  </code></pre>
  
  <p>but I keep getting the following message:</p>
  
  <pre><code>Traceback (most recent call last):
    File "C:..../CloudWatch_GetMetricStatistics.py", line 13, in &lt;module&gt;
      {'InstanceId':'i-0c996c11414476c7c'},
    File "C:\Program Files\Python27\lib\site-packages\botocore\client.py", line 251, in _api_call
      "%s() only accepts keyword arguments." % py_operation_name)
  TypeError: get_metric_statistics() only accepts keyword arguments.
  </code></pre>
  
  <p>I have:</p>
  
  <ol>
  <li>Looked at the documentation on Boto3 and I believe I have got everything correctly written/included</li>
  <li>Set the correct region/output format/security credentials in the .aws folder</li>
  <li>Googled similar problems with put_metric_statistics, etc to try and figure it out</li>
  </ol>
  
  <p>I'm still stuck as to what I'm missing?</p>
  
  <p>Any guidance would be much appreciated.</p>
  
  <p>Many thanks
  Ben</p>