Count the numer of instance in an array using JMESPath
<p>In the example JSON at the bottom of this question, how can I count the number of key/value pairs in the array <code>"Tags"</code> using JMESPath?</p>

<p>According to the <a href="http://jmespath.readthedocs.io/en/stable/specification.html#current-node" rel="noreferrer">JMESPath documentation</a>, I can do this using the <code>count()</code> function -</p>

<blockquote>
  <p>For example, the following expression creates an array containing the total number of elements in the foo object followed by the value of foo["bar"].</p>
</blockquote>

<p>However, it seems that the documentation is incorrect.  Using the JMESPath website, the query <code>Reservations[].Instances[].[count(@), Tags]</code> yeilds the result <code>[ [ null ] ]</code>.  I then tested via the AWS command line and an error was returned -</p>

<blockquote>
  <p>Unknown function: count()</p>
</blockquote>

<p>Is there actually a way of doing this using JMESPath?</p>

<p>Example JSON -</p>

<pre><code>{
    "Reservations": [
        {
            "Instances": [
                {
                    "InstanceId": "i-asdf1234",
                    "InstanceName": "My Instance",
                    "Tags": [
                        {
                            "Value": "Value1",
                            "Key": "Key1"
                        },
                        {
                            "Value": "Value2",
                            "Key": "Key2"
                        },
                        {
                            "Value": "Value3",
                            "Key": "Key3"
                        },
                        {
                            "Value": "Value4",
                            "Key": "Key4"
                        }
                    ]
                }
            ]
        }
    ]
}
</code></pre>