getting "The parameter MetricData is required."

[barrytam20]

I've been getting the following error when using the config below, would you be able to help me out? I'm confused on what value_key should be as it doesnt look to map to anything related to the aws cli command.

I am running this in a container and doing gem install of your plugin, my versions are

2018-07-26 00:40:03 +0000 [info]: starting fluentd-1.2.3 pid=7 ruby="2.4.4"
2018-07-26 00:40:04 +0000 [info]: gem 'fluent-plugin-cloudwatch-put' version '0.2.1'
2018-07-26 00:40:04 +0000 [info]: gem 'fluentd' version '1.2.3'

    @type cloudwatch_put

    <buffer tag>
      flush_interval 20s
    </buffer>

    region us-east-1

    namespace "test-namesapce"
    metric_name ${tag[0]}
    unit Count
    value_key "test"

    storage_resolution 1

    <dimensions>
      name "test"
      value ${tag[0]}
    </dimensions>
  </match>```