Elastic: Fix displaying of correct log message (#26020)

* Fix default field, remove redundant line field check

* Add comments

public/app/core/logs_model.ts

@@ -319,10 +319,7 @@ export function logSeriesToLogsModel(logSeries: DataFrame[]): LogsModel | undefi
   // Find the fields we care about and collect all labels
   const allSeries: LogFields[] = logSeries.map(series => {
     const fieldCache = new FieldCache(series);
-
-    const stringField = fieldCache.hasFieldNamed('line')
-      ? fieldCache.getFieldByName('line')
-      : fieldCache.getFirstFieldOfType(FieldType.string);
+    const stringField = fieldCache.getFirstFieldOfType(FieldType.string);
     if (stringField?.labels) {
       allLabels.push(stringField.labels);
     }

public/app/plugins/datasource/loki/result_transformer.ts

@@ -78,7 +78,7 @@ function constructDataFrame(
     refId,
     fields: [
       { name: 'ts', type: FieldType.time, config: { displayName: 'Time' }, values: times }, // Time
-      { name: 'line', type: FieldType.string, config: {}, values: lines, labels }, // Line
+      { name: 'line', type: FieldType.string, config: {}, values: lines, labels }, // Line - needs to be the first field with string type
       { name: 'id', type: FieldType.string, config: {}, values: uids },
       { name: 'tsNs', type: FieldType.time, config: { displayName: 'Time ns' }, values: timesNs }, // Time
     ],