add num_review_comments (GMD) metric implementation

Author: vchrombie

grimoire_elk/enriched/gitlabcomments.py

@@ -367,6 +367,7 @@ def __get_rich_merge(self, item):
         rich_mr['code_merge_duration'] = get_time_diff_days(merge_request['created_at'],
                                                             merge_request['merged_at'])
         rich_mr['num_versions'] = len(merge_request['versions_data'])
+        rich_mr['num_review_comments'] = len(merge_request['notes_data'])
 
         rich_mr['time_to_merge_request_response'] = None
         if merge_request['notes_data'] != 0: