[Backport 2.x] Add getMetadataFields to MapperService (#13819)

CHANGELOG.md

@@ -10,6 +10,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Make outbound side of transport protocol dependent ([#13293](https://github.com/opensearch-project/OpenSearch/pull/13293))
 - [Remote Store] Upload translog checkpoint as object metadata to translog.tlog([#13637](https://github.com/opensearch-project/OpenSearch/pull/13637))
 - [Remote Store] Add dynamic cluster settings to set timeout for segments upload to Remote Store ([#13679](https://github.com/opensearch-project/OpenSearch/pull/13679))
+- Add getMetadataFields to MapperService ([#13819](https://github.com/opensearch-project/OpenSearch/pull/13819))
 
 ### Dependencies
 - Bump `com.github.spullara.mustache.java:compiler` from 0.9.10 to 0.9.13 ([#13329](https://github.com/opensearch-project/OpenSearch/pull/13329), [#13559](https://github.com/opensearch-project/OpenSearch/pull/13559))

server/src/main/java/org/opensearch/index/mapper/MapperService.java

@@ -710,6 +710,13 @@ public boolean isMetadataField(String field) {
         return mapperRegistry.isMetadataField(indexVersionCreated, field);
     }
 
+    /**
+     * Returns a set containing the registered metadata fields
+     */
+    public Set<String> getMetadataFields() {
+        return Collections.unmodifiableSet(mapperRegistry.getMetadataMapperParsers(indexVersionCreated).keySet());
+    }
+
     /**
      * An analyzer wrapper that can lookup fields within the index mappings
      */

server/src/test/java/org/opensearch/index/mapper/MapperServiceTests.java

@@ -52,6 +52,7 @@
 import org.opensearch.index.mapper.KeywordFieldMapper.KeywordFieldType;
 import org.opensearch.index.mapper.MapperService.MergeReason;
 import org.opensearch.index.mapper.NumberFieldMapper.NumberFieldType;
+import org.opensearch.indices.IndicesModule;
 import org.opensearch.indices.InvalidTypeNameException;
 import org.opensearch.indices.analysis.AnalysisModule.AnalysisProvider;
 import org.opensearch.plugins.AnalysisPlugin;
@@ -87,6 +88,11 @@ public void testTypeValidation() {
         MapperService.validateTypeName("_doc"); // no exception
     }
 
+    public void testGetMetadataFieldsReturnsExpectedSet() throws Throwable {
+        final MapperService mapperService = createIndex("test1").mapperService();
+        assertEquals(mapperService.getMetadataFields(), IndicesModule.getBuiltInMetadataFields());
+    }
+
     public void testPreflightUpdateDoesNotChangeMapping() throws Throwable {
         final MapperService mapperService = createIndex("test1").mapperService();
         final CompressedXContent mapping = createMappingSpecifyingNumberOfFields(1);