[metrics] make 'merged_entities_count_of_tablet' visible when only on…

…e entity merged

Before this patch, the metric 'merged_entities_count_of_tablet' is hidden
when tablet server aggregate metrics by the same attribute and there is
only one entity merged, because it's modification epoch is not updated.
We should expose the metric in this condition.

Change-Id: Ia74cae25b5bde7d61db1186e44abdff487d3f23f
Reviewed-on: http://gerrit.cloudera.org:8080/15707
Tested-by: Kudu Jenkins
Reviewed-by: Alexey Serbin <[email protected]>

src/kudu/tserver/tablet_server-test.cc

@@ -659,6 +659,19 @@ TEST_F(TabletServerTest, TestTombstonedTabletOnWebUI) {
   ASSERT_STR_NOT_CONTAINS(s, mini_server_->bound_rpc_addr().ToString());
 }
 
+// When tablet server merge metrics by the same attributes, the metric
+// 'merged_entities_count_of_tablet' should be visible
+TEST_F(TabletServerTest, TestMergedEntitiesCount) {
+  EasyCurl c;
+  faststring buf;
+  const string addr = mini_server_->bound_http_addr().ToString();
+  ASSERT_OK(c.FetchURL(Substitute("http://$0/metrics", addr), &buf));
+  ASSERT_STR_NOT_CONTAINS(buf.ToString(), "merged_entities_count_of_tablet");
+  ASSERT_OK(c.FetchURL(Substitute("http://$0/metrics?merge_rules=tablet|table|table_name", addr),
+                       &buf));
+  ASSERT_STR_CONTAINS(buf.ToString(), "merged_entities_count_of_tablet");
+}
+
 class TabletServerDiskSpaceTest : public TabletServerTestBase,
                                   public testing::WithParamInterface<string> {
  public:

src/kudu/util/metrics.cc

@@ -426,6 +426,9 @@ Status MetricEntity::CollectTo(MergedEntityMetrics* collections,
     scoped_refptr<Metric> entry = FindPtrOrNull(entity_collection, prototype);
     if (!entry) {
       scoped_refptr<Metric> new_metric = metric->snapshot();
+      if (!new_metric->invalid_for_merge_) {
+        new_metric->UpdateModificationEpoch();
+      }
       InsertOrDie(&entity_collection, new_metric->prototype(), new_metric);
     } else {
       entry->MergeFrom(metric);