[skip ci] Add comments of locust file on benchmark (#9568)

Signed-off-by: Nwangting0128 <ting.wang@zilliz.com>

tests/benchmark/milvus_benchmark/runners/locust.py

@@ -94,6 +94,7 @@ class LocustInsertRunner(LocustRunner):
         }
         self.init_metric(self.name, collection_info, index_info, None, run_params)
         case_metric = copy.deepcopy(self.metric)
+        # set metric type as case
         case_metric.set_case_metric_type()
         case_metrics = list()
         case_params = list()