Merge pull request #1142 from GoogleChrome/20210114-fix-metrics-fetch-flask-return

Return something from Yesterday handler to satisfy Flask.

admin.py

@@ -159,6 +159,8 @@ class YesterdayHandler(common.FlaskHandler):
             'Got error %s while fetching usage data' % response_code)
         return error_message, 500
 
+    return 'Success'
+
 
 class HistogramsHandler(common.FlaskHandler):