Disable downsampling force merge via mapping metadata (#135694)

In this PR we introduce a mapping metadata field that allows us to skip
the force-merge request during a downsamping API request.

When an index has the `downsample.forcemerge.enabled` in their mapping
metadata, they can enable or disable the force merge:

```
{
  "_meta": {
      "downsample.forcemerge.enabled": false
  }
}
```

By default, force merge is enabled to match the current user experience.

Author: gmarouli

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

@@ -31,6 +31,7 @@
 import java.io.IOException;
 import java.time.Instant;
 import java.util.List;
+import java.util.Locale;
 import java.util.concurrent.TimeUnit;
 import java.util.function.Supplier;
 
@@ -44,8 +45,9 @@ public class DownsampleIT extends DownsamplingIntegTestCase {
 
     public void testDownsamplingPassthroughDimensions() throws Exception {
         String dataStreamName = "metrics-foo";
-        String mapping = """
+        String mapping = String.format(Locale.ROOT, """
             {
+              %s
               "properties": {
                 "attributes": {
                   "type": "passthrough",
@@ -64,7 +66,7 @@ public void testDownsamplingPassthroughDimensions() throws Exception {
                 }
               }
             }
-            """;
+            """, generateForceMergeMetadata());
 
         // Create data stream by indexing documents
         final Instant now = Instant.now();
@@ -171,6 +173,16 @@ private void downsampleAndAssert(String dataStreamName, String mapping, Supplier
         assertDownsampleIndexFieldsAndDimensions(sourceIndex, targetIndex, downsampleConfig);
     }
 
+    private String generateForceMergeMetadata() {
+        return switch (randomIntBetween(0, 4)) {
+            case 0 -> "\"_meta\": { \"downsample.forcemerge.enabled\": false},";
+            case 1 -> "\"_meta\": { \"downsample.forcemerge.enabled\": true},";
+            case 2 -> "\"_meta\": { \"downsample.forcemerge.enabled\": 4},";
+            case 3 -> "\"_meta\": { \"downsample.forcemerge.enabled\": null},";
+            default -> "";
+        };
+    }
+
     public void testAggMetricInEsqlTSAfterDownsampling() throws Exception {
         String dataStreamName = "metrics-foo";
         Settings settings = Settings.builder().put("mode", "time_series").putList("routing_path", List.of("host", "cluster")).build();

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

@@ -264,6 +264,8 @@ void assertDownsampleIndexFieldsAndDimensions(String sourceIndex, String downsam
         final CompressedXContent sourceIndexCompressedXContent = new CompressedXContent(sourceIndexMappings);
         mapperService.merge(MapperService.SINGLE_MAPPING_NAME, sourceIndexCompressedXContent, MapperService.MergeReason.INDEX_TEMPLATE);
 
+        assertThat(downsampleIndexMappings.get("_meta"), equalTo(sourceIndexMappings.get("_meta")));
+
         // Collect expected mappings for fields and dimensions
         Map<String, TimeSeriesParams.MetricType> metricFields = new HashMap<>();
         Map<String, String> dimensionFields = new HashMap<>();