Create a cloudwatch reporter to expose scale metric

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

Or with SDK1
https://docs.aws.amazon.com/sdk-for-java/v1/developer-guide/examples-cloudwatch-publish-custom-metrics.html
https://github.com/awsdocs/aws-doc-sdk-examples/blob/f4eaf2b2971805cfb2b87a8e5ab408f83169432e/java/example_code/cloudwatch/src/main/java/aws/example/cloudwatch/PutMetricData.java