Support aggregate_double_metric fields in the Field API

x-pack/plugin/mapper-aggregate-metric/src/main/java/org/elasticsearch/xpack/aggregatemetric/mapper/AggregateDoubleMetricFieldMapper.java

@@ -493,18 +493,7 @@ public BucketedSort newBucketedSort(
 
         @Override
         public ValueFetcher valueFetcher(SearchExecutionContext context, String format) {
-            if (format != null) {
-                throw new IllegalArgumentException("Field [" + name() + "] of type [" + typeName() + "] doesn't support formats.");
-            }
-
-            return new SourceValueFetcher(name(), context) {
-                @Override
-                @SuppressWarnings("unchecked")
-                protected Object parseSourceValue(Object value) {
-                    Map<String, Double> metrics = (Map<String, Double>) value;
-                    return metrics.get(defaultMetric.name());
-                }
-            };
+            return SourceValueFetcher.identity(name(), context, format);
         }
 
         /**

x-pack/plugin/mapper-aggregate-metric/src/test/java/org/elasticsearch/xpack/aggregatemetric/mapper/AggregateDoubleMetricFieldTypeTests.java

@@ -83,16 +83,17 @@ public void testRangeQuery() {
 
     public void testFetchSourceValueWithOneMetric() throws IOException {
         final MappedFieldType fieldType = createDefaultFieldType("field", Collections.emptyMap(), Metric.min);
-        final double defaultValue = 45.8;
-        final Map<String, Object> metric = Collections.singletonMap("min", defaultValue);
-        assertEquals(List.of(defaultValue), fetchSourceValue(fieldType, metric));
+        final double min = 45.8;
+        final Map<String, Object> metric = Collections.singletonMap("min", min);
+        assertEquals(List.of(metric), fetchSourceValue(fieldType, metric));
     }
 
     public void testFetchSourceValueWithMultipleMetrics() throws IOException {
         final MappedFieldType fieldType = createDefaultFieldType("field", Collections.emptyMap(), Metric.max);
-        final double defaultValue = 45.8;
-        final Map<String, Object> metric = Map.of("min", 14.2, "max", defaultValue);
-        assertEquals(List.of(defaultValue), fetchSourceValue(fieldType, metric));
+        final double max = 45.8;
+        final double min = 14.2;
+        final Map<String, Object> metric = Map.of("min", min, "max", max);
+        assertEquals(List.of(metric), fetchSourceValue(fieldType, metric));
     }
 
     /** Tests that aggregate_metric_double uses the default_metric subfield's doc-values as values in scripts */

x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/aggregate-metrics/10_basic.yml

@@ -290,3 +290,63 @@
         index: test
         body:
           sort: [ { metric.min: desc } ]
+
+---
+"Test fields api":
+  - skip:
+      version: "- 8.3.99"
+      reason: "Breaking change introduced in 8.4.0"
+  - do:
+      indices.create:
+        index:  test
+        body:
+          mappings:
+            properties:
+              metric:
+                type: aggregate_metric_double
+                metrics: [min, max, sum, value_count]
+                default_metric: sum
+
+  - do:
+      index:
+        index:  test
+        id:     "1"
+        body:
+          metric:
+            min: 10
+            max: 100
+            sum: 200
+            value_count: 5
+        refresh: true
+
+  - do:
+      index:
+        index:  test
+        id:     "2"
+        body:
+          metric:
+            min: 50
+            max: 1000
+            sum: 5000
+            value_count: 10
+        refresh: true
+
+  - do:
+      search:
+        index: test
+        body:
+          _source: false
+          fields: [ metric ]
+          query:
+            match_all: {}
+
+  - match: { hits.total.value: 2 }
+  - length: { hits.hits: 2 }
+  - match: { hits.hits.0.fields.metric.0.min: 10 }
+  - match: { hits.hits.0.fields.metric.0.max: 100 }
+  - match: { hits.hits.0.fields.metric.0.sum: 200 }
+  - match: { hits.hits.0.fields.metric.0.value_count: 5 }
+  - match: { hits.hits.1.fields.metric.0.min: 50 }
+  - match: { hits.hits.1.fields.metric.0.max: 1000 }
+  - match: { hits.hits.1.fields.metric.0.sum: 5000 }
+  - match: { hits.hits.1.fields.metric.0.value_count: 10 }