[ML] Reduce warning logging from get categories Grok pattern creation (…

…#73373)

The Grok pattern creator used within the get categories action
works by looking at the examples for each category. Sometimes
these examples are truncated, and cannot be used for Grok pattern
determination. Previously when this happened we would log a warning,
but that caused a feedback loop in the case where the Elasticsearch
logs themselves were being categorized: the warning messages would
end up creating new categories in the Elasticsearch log categorization,
with very long examples that would be truncated, leading to the same
problem occurring yet again.

This change reduces the warning log to trace for truncated log
messages, and also removes the example from the log message to
shorten it.

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/job/categorization/GrokPatternCreator.java

@@ -8,6 +8,7 @@
 
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
+import org.apache.logging.log4j.message.ParameterizedMessage;
 import org.elasticsearch.grok.Grok;
 
 import java.util.ArrayList;
@@ -124,7 +125,12 @@ public static String findBestGrokMatchFromExamples(String jobId, String regex, C
                 // the message was very long, and the example was truncated.  In this
                 // case we will have appended an ellipsis to indicate truncation.
                 assert example.endsWith("...") : exampleProcessor.pattern() + " did not match non-truncated example " + example;
-                logger.warn("[{}] Pattern [{}] did not match example [{}]", jobId, exampleProcessor.pattern(), example);
+                if (example.endsWith("...")) {
+                    logger.trace(() -> new ParameterizedMessage("[{}] Pattern [{}] did not match truncated example",
+                        jobId, exampleProcessor.pattern()));
+                } else {
+                    logger.warn("[{}] Pattern [{}] did not match non-truncated example [{}]", jobId, exampleProcessor.pattern(), example);
+                }
             }
         }