Allow adding story keys to values with existing tir labels

telemetry/telemetry/internal/results/page_test_results.py

Index: telemetry/telemetry/internal/results/page_test_results.py
diff --git a/telemetry/telemetry/internal/results/page_test_results.py b/telemetry/telemetry/internal/results/page_test_results.py
index d32a5772018cbbec2cfb34889f648192e22de844..83016e545ce068e69a2c20b5fc77d01863faee54 100644
--- a/telemetry/telemetry/internal/results/page_test_results.py
+++ b/telemetry/telemetry/internal/results/page_test_results.py
@@ -179,18 +179,18 @@ class PageTestResults(object):
     story_keys = self._current_page_run.story.grouping_keys
 
     if story_keys:
-      assert not value.tir_label, (
-          'Trying to use a TBMv2 story (with grouping keys) with a TBMv1 metric'
-      )
-
       for k, v in story_keys.iteritems():
         assert k not in value.grouping_keys, (
             'Tried to add story grouping key ' + k + ' already defined by ' +
             'value')
         value.grouping_keys[k] = v
 
-      # We sort by key name to make building the tir_label deterministic.
-      value.tir_label = '_'.join(v for _, v in sorted(story_keys.iteritems()))
+      if value.tir_label:
+        logging.warning('Value already has a tir_label (%s) so story_keys (%s) '
+                        'are ignored', value.tir_label, story_keys)
+      else:
+        # We sort by key name to make building the tir_label deterministic.
+        value.tir_label = '_'.join(v for _, v in sorted(story_keys.iteritems()))
 
     if not (isinstance(value, skip.SkipValue) or
             isinstance(value, failure.FailureValue) or