API key double encoded in Elasticsearch metricset

https://github.com/elastic/beats/blame/d8606faa49423044c9f1952b57fd486ea99c7275/metricbeat/module/elasticsearch/metricset.go#L112

A option was added to the Elasticsearch metricset to retrieve an Elasticsearch API key from the environment and use that in an `Authorization` header after base64 encoding it.

However, API keys returned by Elasticsearch are already base64 encoded. This leads to double encoding the key. Requests to Elasticsearch will subsequently be rejected with HTTP 401.

Start at metricbeat/module/elasticsearch/metricset.go around line 112 and trace how the environment-provided API key becomes the Authorization header. Confirm requests use the already base64-encoded key only once and verify Elasticsearch metricset requests no longer receive HTTP 401 responses.

Written by the indexing model from the issue text.