Ensure ElasticSearch Zone Awareness is enabled

Disabling the Zone Awareness feature may lead to less resilient ElasticSearch clusters.

Zone awareness applies only to multi-az configurations.

In AWS Console -

1. Sign in to AWS Console and go to the Elasticsearch (ES) dashboard.
2. Click on the ES domain.
3. Open the cluster configuration page.
4. In the Node configuration section verify if zone awareness is enabled.

In Terraform -

1. In the aws_elasticsearch_domain resource, set the cluster_config.zone_awareness_enabled field to true.
2. Configure a zone_awareness_config block with the availability_zone_count field set to the value representing the number of zones.

References:
https://docs.aws.amazon.com/opensearch-service/latest/developerguide/managedomains-multiaz.html
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/elasticsearch_domain#zone_awareness_enabled