Ignore JSON logs for merge parts analysis

With this commit we ignore any JSON log files when analyzing merge
parts. Because Elasticsearch writes both plain text and JSON log files
with elastic/elasticsearch#36833, we need to ignore one of them in order
to avoid double-counting. We choose to ignore the JSON files for now as
this is backwards-compatible.

Relates #637
Relates elastic/elasticsearch#36833

esrally/mechanic/telemetry.py

@@ -653,12 +653,14 @@ def on_benchmark_stop(self):
         merge_times = {}
         for log_file in os.listdir(self.node_log_dir):
             log_path = "%s/%s" % (self.node_log_dir, log_file)
-            if not io.is_archive(log_file):
+            if io.is_archive(log_file):
+                self.logger.debug("Skipping archived logs in [%s].", log_path)
+            elif io.has_extension(log_file, ".json"):
+                self.logger.debug("Skipping JSON-formatted logs in [%s].", log_path)
+            else:
                 self.logger.debug("Analyzing merge times in [%s]", log_path)
                 with open(log_path, mode="rt", encoding="utf-8") as f:
                     self._extract_merge_times(f, merge_times)
-            else:
-                self.logger.debug("Skipping archived logs in [%s].", log_path)
         if merge_times:
             self._store_merge_times(merge_times)
         self.logger.info("Finished analyzing merge times. Extracted [%s] different merge time components.", len(merge_times))