Elasticsearch: Visualize logs in Explore (#17605)

* explore: try to use existing mode when switching datasource

* elasticsearch: initial explore logs support

* Elasticsearch: Adds ElasticsearchOptions type
Updates tests accordingly

* Elasticsearch: Adds typing to query method

* Elasticsearch: Makes maxConcurrentShardRequests optional

* Explore: Allows empty query for elasticsearch datasource

* Elasticsearch: Unifies ElasticsearchQuery interface definition
Removes check for context === 'explore'

* Elasticsearch: Removes context property from ElasticsearchQuery interface
Adds field property
Removes metricAggs property
Adds typing to metrics property

* Elasticsearch: Runs default 'empty' query when 'clear all' button is pressed

* Elasticsearch: Removes index property from ElasticsearchOptions interface

* Elasticsearch: Removes commented code from ElasticsearchQueryField.tsx

* Elasticsearch: Adds comment warning usage of for...in to elastic_response.ts

* Elasticsearch: adds tests related to log queries

pkg/api/frontendsettings.go

@@ -115,11 +115,7 @@ func (hs *HTTPServer) getFrontendSettingsMap(c *m.ReqContext) (map[string]interf
 			}
 		}
 
-		if ds.Type == m.DS_ES {
-			dsMap["index"] = ds.Database
-		}
-
-		if ds.Type == m.DS_INFLUXDB {
+		if (ds.Type == m.DS_INFLUXDB) || (ds.Type == m.DS_ES) {
 			dsMap["database"] = ds.Database
 		}