UKM export=false belongs on <statistics> tag.

Putting the export=false on the <enumeration> tag ends up just being
ignored.  Local-metric aggregation works fine but the key-metric ends
up being also aggregated and stored in the output tables.

Change-Id: I3a061f14af847bb55bbd49d5fe0c8a918a4017cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566390Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650461}

tools/metrics/ukm/ukm.xml

@@ -3743,8 +3743,8 @@ be describing additional metrics about the same event.
     </summary>
     <aggregation>
       <history>
-        <statistics>
-          <enumeration export="False"/>
+        <statistics export="False">
+          <enumeration/>
         </statistics>
       </history>
     </aggregation>

tools/metrics/ukm/ukm_model.py

@@ -18,16 +18,13 @@ _LOWERCASE_NAME_FN = lambda n: n.get('name').lower()
 
 _ENUMERATION_TYPE = models.ObjectNodeType(
     'enumeration',
-    attributes=[
-      ('export', unicode, r'^(?i)(|true|false)$'),
-    ],
+    attributes=[],
     single_line=True)
 
 _QUANTILES_TYPE = models.ObjectNodeType(
     'quantiles',
     attributes=[
       ('type', unicode, None),
-      ('export', unicode, r'^(?i)(|true|false)$'),
     ],
     single_line=True)
 
@@ -40,7 +37,9 @@ _INDEX_TYPE = models.ObjectNodeType(
 
 _STATISTICS_TYPE =  models.ObjectNodeType(
     'statistics',
-    attributes=[],
+    attributes=[
+      ('export', unicode, r'^(?i)(|true|false)$'),
+    ],
     children=[
         models.ChildType('quantiles', _QUANTILES_TYPE, multiple=False),
         models.ChildType('enumeration', _ENUMERATION_TYPE, multiple=False),

tools/metrics/ukm/ukm_model_test.py

@@ -37,8 +37,8 @@ PRETTY_XML = """
   <metric name="Metric2">
     <aggregation>
       <history>
-        <statistics>
-          <enumeration export="False"/>
+        <statistics export="False">
+          <enumeration/>
         </statistics>
       </history>
     </aggregation>