Allow scripted metric agg to access `_score` (#24295)

* Fixes #24259

Corrects the ScriptedMetricAggregator so that the script can have
access to scores during the map stage.

* Restored original tests. Added seperate test.

As requested, I've restored the non-score dependant tests, and added the
score dependent metric as a seperate test.

core/src/main/java/org/elasticsearch/search/aggregations/metrics/scripted/ScriptedMetricAggregator.java

@@ -63,7 +63,7 @@ public class ScriptedMetricAggregator extends MetricsAggregator {
     public LeafBucketCollector getLeafCollector(LeafReaderContext ctx,
             final LeafBucketCollector sub) throws IOException {
         final LeafSearchScript leafMapScript = mapScript.getLeafSearchScript(ctx);
-        return new LeafBucketCollectorBase(sub, mapScript) {
+        return new LeafBucketCollectorBase(sub, leafMapScript) {
             @Override
             public void collect(int doc, long bucket) throws IOException {
                 assert bucket == 0 : bucket;

core/src/test/java/org/elasticsearch/search/aggregations/metrics/scripted/ScriptedMetricAggregatorTests.java

@@ -33,6 +33,7 @@ import org.elasticsearch.index.mapper.MappedFieldType;
 import org.elasticsearch.index.query.QueryShardContext;
 import org.elasticsearch.indices.breaker.CircuitBreakerService;
 import org.elasticsearch.script.MockScriptEngine;
+import org.elasticsearch.script.ScoreAccessor;
 import org.elasticsearch.script.Script;
 import org.elasticsearch.script.ScriptContextRegistry;
 import org.elasticsearch.script.ScriptEngineRegistry;
@@ -59,6 +60,11 @@ public class ScriptedMetricAggregatorTests extends AggregatorTestCase {
     private static final Script MAP_SCRIPT = new Script(ScriptType.INLINE, MockScriptEngine.NAME, "mapScript", Collections.emptyMap());
     private static final Script COMBINE_SCRIPT = new Script(ScriptType.INLINE, MockScriptEngine.NAME, "combineScript",
             Collections.emptyMap());
+
+    private static final Script INIT_SCRIPT_SCORE = new Script(ScriptType.INLINE, MockScriptEngine.NAME, "initScriptScore", Collections.emptyMap());
+    private static final Script MAP_SCRIPT_SCORE = new Script(ScriptType.INLINE, MockScriptEngine.NAME, "mapScriptScore", Collections.emptyMap());
+    private static final Script COMBINE_SCRIPT_SCORE = new Script(ScriptType.INLINE, MockScriptEngine.NAME, "combineScriptScore",
+            Collections.emptyMap());
     private static final Map<String, Function<Map<String, Object>, Object>> SCRIPTS = new HashMap<>();
 
 
@@ -79,6 +85,21 @@ public class ScriptedMetricAggregatorTests extends AggregatorTestCase {
             Map<String, Object> agg = (Map<String, Object>) params.get("_agg");
             return ((List<Integer>) agg.get("collector")).stream().mapToInt(Integer::intValue).sum();
         });
+
+        SCRIPTS.put("initScriptScore", params -> {
+            Map<String, Object> agg = (Map<String, Object>) params.get("_agg");
+            agg.put("collector", new ArrayList<Double>());
+            return agg;
+            });
+        SCRIPTS.put("mapScriptScore", params -> {
+            Map<String, Object> agg = (Map<String, Object>) params.get("_agg");
+            ((List<Double>) agg.get("collector")).add(((ScoreAccessor) params.get("_score")).doubleValue());
+            return agg;
+        });
+        SCRIPTS.put("combineScriptScore", params -> {
+            Map<String, Object> agg = (Map<String, Object>) params.get("_agg");
+            return ((List<Double>) agg.get("collector")).stream().mapToDouble(Double::doubleValue).sum();
+        });
     }
 
     @SuppressWarnings("unchecked")
@@ -144,6 +165,29 @@ public class ScriptedMetricAggregatorTests extends AggregatorTestCase {
         }
     }
 
+    /**
+     * test that uses the score of the documents
+     */
+    public void testScriptedMetricWithCombineAccessesScores() throws IOException {
+        try (Directory directory = newDirectory()) {
+            Integer numDocs = randomInt(100);
+            try (RandomIndexWriter indexWriter = new RandomIndexWriter(random(), directory)) {
+                for (int i = 0; i < numDocs; i++) {
+                    indexWriter.addDocument(singleton(new SortedNumericDocValuesField("number", i)));
+                }
+            }
+            try (IndexReader indexReader = DirectoryReader.open(directory)) {
+                ScriptedMetricAggregationBuilder aggregationBuilder = new ScriptedMetricAggregationBuilder(AGG_NAME);
+                aggregationBuilder.initScript(INIT_SCRIPT_SCORE).mapScript(MAP_SCRIPT_SCORE).combineScript(COMBINE_SCRIPT_SCORE);
+                ScriptedMetric scriptedMetric = search(newSearcher(indexReader, true, true), new MatchAllDocsQuery(), aggregationBuilder);
+                assertEquals(AGG_NAME, scriptedMetric.getName());
+                assertNotNull(scriptedMetric.aggregation());
+                // all documents have score of 1.0
+                assertEquals((double) numDocs, scriptedMetric.aggregation());
+            }
+        }
+    }
+
     /**
      * We cannot use Mockito for mocking QueryShardContext in this case because
      * script-related methods (e.g. QueryShardContext#getLazyExecutableScript)