[ML] Add logging around flushing input stream in DFA data loading (#72236)

Relates #70698

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/dataframe/process/AnalyticsProcessManager.java

@@ -177,7 +177,9 @@ public class AnalyticsProcessManager {
             writeHeaderRecord(dataExtractor, process, task);
             writeDataRows(dataExtractor, process, task);
             process.writeEndOfDataMessage();
+            LOGGER.debug(() -> new ParameterizedMessage("[{}] Flushing input stream", processContext.config.getId()));
             process.flushStream();
+            LOGGER.debug(() -> new ParameterizedMessage("[{}] Flushing input stream completed", processContext.config.getId()));
 
             restoreState(config, process, hasState);