Variables: Elasticsearch datasource ignores multiple field parameters in variables query

### What happened? When creating a dashboard variable with an Elasticsearch query containing multiple field parameters, Grafana uses only the last specified field and ignores previous ones. Example query: { "find": "terms", "query": "(_index:index_A OR _index:index_B)", "field": "field_A", "field": "field_B" // Only this field is used! } ### What did you expect to happen? Expected behavior: Grafana should combine values from all specified fields (field_A + field_B), similar to how multi_fields works in Elasticsearch. Alternatively, it should throw a validation error if multiple field parameters are not supported. ### Did this work before? Unknown (behavior observed in Grafana 9.5.2). Need confirmation from maintainers. ### How do we reproduce it? 1.Create a variable with type Query and Elasticsearch datasource. 2.Use the JSON query above (with two field parameters). 3.Check "Preview of values" — only values from the last field will appear. ### Is the bug inside a dashboard panel? No, used in variables ### Environment (with versions)? Grafana: 9.5.2 Elasticsearch 8.7.1 ### Grafana platform? Docker ### Datasource(s)? Elasticsearch 8.7.1