Fix bug introduced when we fixed multi-fields in #138869 (#139104) (#139111)

In #138869 we fixed the handling of the multi-fields; however, we
introduced a bug when a mapping contains a alias type.

For example, the following mapping:

```
"@timestamp": {
  "type": "date"
},
"timestamp": {
  "type": "alias",
  "path": "@timestamp"
}
```

The cause of the bug was that in the `FieldValueFetcher` we switch the
check wether a field is indexed from the name we retrieved in the label
to the name of the field type. In this case, the label is `timestamp`
which does not in the indexed but the name of the field type is
"@timestamp".

(cherry picked from commit 47698be)

# Conflicts:
#	x-pack/plugin/downsample/src/internalClusterTest/java/org/elasticsearch/xpack/downsample/DownsampleIT.java

Author: gmarouli

x-pack/plugin/downsample/src/internalClusterTest/java/org/elasticsearch/xpack/downsample/DownsampleIT.java

@@ -49,6 +49,13 @@ public void testDownsamplingPassthroughDimensions() throws Exception {
             {
               %s
               "properties": {
+                "@timestamp": {
+                  "type": "date"
+                },
+                "timestamp": {
+                  "path": "@timestamp",
+                  "type": "alias"
+                },
                 "attributes": {
                   "type": "passthrough",
                   "priority": 10,
@@ -92,6 +99,13 @@ public void testDownsamplingPassthroughMetrics() throws Exception {
         String mapping = """
             {
               "properties": {
+                "@timestamp": {
+                  "type": "date"
+                },
+                "timestamp": {
+                  "path": "@timestamp",
+                  "type": "alias"
+                },
                 "attributes.os.name": {
                   "type": "keyword",
                   "time_series_dimension": true

x-pack/plugin/downsample/src/main/java/org/elasticsearch/xpack/downsample/FieldValueFetcher.java

@@ -101,7 +101,7 @@ static List<FieldValueFetcher> create(SearchExecutionContext context, String[] f
                     }
                 }
             } else {
-                if (context.fieldExistsInIndex(fieldType.name())) {
+                if (context.fieldExistsInIndex(field)) {
                     final IndexFieldData<?> fieldData;
                     if (fieldType instanceof FlattenedFieldMapper.RootFlattenedFieldType flattenedFieldType) {
                         var keyedFieldType = flattenedFieldType.getKeyedFieldType();