Instead of having a full Elasticsearch client configuration (url, username, password, ca), we could let the user set a reference to an Elasticsearch cluster managed by ECK.
It would allow the metrics adapter to setup automatically the URL and the certificates. We could use the elastic user in a first step, I don't think it would be easy to automatically setup a dedicated Elasticsearch user with the right set of privileges.
One downside of this feature is that it would require the metrics adapter to read Secrets which are likely to exist in a different namespace.