remove ElasticSearch MasterFreeStorageSpace metric (#1059)
remove ElasticSearch MasterFreeStorageSpace metric (#1059)

From the ElasticSearch CloudWatch metrics documentation:

https://docs.aws.amazon.com/elasticsearch-service/latest/developerguide/es-managedomains.html#es-managedomains-cloudwatchmetrics-master-node-metrics

This metric is not relevant and can be ignored. The service does not use
master nodes as data nodes.
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