[ML] Add debug logging for field caps request during DF Analytics (#61459)

Adds debug logging for the request and the response that is getting
field capabilities during a data frame analytics job.

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/dataframe/extractor/ExtractedFieldsDetectorFactory.java

@@ -6,6 +6,9 @@
 package org.elasticsearch.xpack.ml.dataframe.extractor;
 
 import com.carrotsearch.hppc.cursors.ObjectObjectCursor;
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+import org.apache.logging.log4j.message.ParameterizedMessage;
 import org.elasticsearch.ResourceNotFoundException;
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.action.admin.indices.settings.get.GetSettingsRequest;
@@ -32,6 +35,7 @@ import org.elasticsearch.xpack.core.ml.dataframe.DataFrameAnalyticsConfig;
 import org.elasticsearch.xpack.core.ml.dataframe.analyses.FieldCardinalityConstraint;
 import org.elasticsearch.xpack.core.ml.utils.ExceptionsHelper;
 
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.Iterator;
@@ -46,6 +50,8 @@ import java.util.concurrent.atomic.AtomicReference;
  */
 public class ExtractedFieldsDetectorFactory {
 
+    private static final Logger LOGGER = LogManager.getLogger(ExtractedFieldsDetectorFactory.class);
+
     private final Client client;
 
     public ExtractedFieldsDetectorFactory(Client client) {
@@ -77,6 +83,8 @@ public class ExtractedFieldsDetectorFactory {
         // Step 3. Get cardinalities for fields with constraints
         ActionListener<FieldCapabilitiesResponse> fieldCapabilitiesHandler = ActionListener.wrap(
             fieldCapabilitiesResponse -> {
+                LOGGER.debug(() -> new ParameterizedMessage(
+                    "[{}] Field capabilities response: {}", config.getId(), fieldCapabilitiesResponse));
                 fieldCapsResponseHolder.set(fieldCapabilitiesResponse);
                 getCardinalitiesForFieldsWithConstraints(index, config, fieldCapabilitiesResponse, fieldCardinalitiesHandler);
             },
@@ -158,6 +166,8 @@ public class ExtractedFieldsDetectorFactory {
         fieldCapabilitiesRequest.indices(index);
         fieldCapabilitiesRequest.indicesOptions(IndicesOptions.lenientExpandOpen());
         fieldCapabilitiesRequest.fields("*");
+        LOGGER.debug(() -> new ParameterizedMessage(
+            "[{}] Requesting field caps for index {}", config.getId(), Arrays.toString(index)));
         ClientHelper.executeWithHeaders(config.getHeaders(), ClientHelper.ML_ORIGIN, client, () -> {
             client.execute(FieldCapabilitiesAction.INSTANCE, fieldCapabilitiesRequest, listener);
             // This response gets discarded - the listener handles the real response