[RelVal] Fix index (AliceO2Group#1456)

Author: benedikt-voelkel

RelVal/utils/o2dpg_release_validation_utils.py

@@ -400,7 +400,7 @@ def load(self, summaries_to_test):
                 idx, metric = self.get_metric_checking_dict(line)
                 if idx is None:
                     # in this case, this metric is new
-                    idx = len(self.metrics) - 1
+                    idx = len(self.metrics)
                     if not self.add_metric(metric):
                         # only attempt to add if that metric is not yet there
                         continue