Create a cloudwatch metric reporter

It's not enough to have a specific Nuxeo metric (NXP-31679) that can be used to auto scale.

This metric must be accessible by the auto scaler, in AWS this means having a cloudwatch metric.

Instead of adding glue to get and publish the Nuxeo scale metric, it will be better to simply have a cloudwatch metric reporter in Nuxeo.

See:

https://docs.aws.amazon.com/sdk-for-java/latest/developer-guide/examples-cloudwatch-publish-custom-metrics.html