Update gauges when a subset LB is destroyed (#3917)

Without these updates, the gauges will be incorrect after a hot
restart.

Fixes #3916

source/common/upstream/subset_lb.cc

@@ -54,7 +54,17 @@ SubsetLoadBalancer::SubsetLoadBalancer(
       });
 }
 
-SubsetLoadBalancer::~SubsetLoadBalancer() { original_priority_set_callback_handle_->remove(); }
+SubsetLoadBalancer::~SubsetLoadBalancer() {
+  original_priority_set_callback_handle_->remove();
+
+  // Ensure gauges reflect correct values.
+  forEachSubset(subsets_, [&](LbSubsetEntryPtr entry) {
+    if (entry->initialized() && entry->active()) {
+      stats_.lb_subsets_removed_.inc();
+      stats_.lb_subsets_active_.dec();
+    }
+  });
+}
 
 void SubsetLoadBalancer::refreshSubsets() {
   for (auto& host_set : original_priority_set_.hostSetsPerPriority()) {

test/common/upstream/subset_lb_test.cc

@@ -1433,6 +1433,26 @@ TEST_F(SubsetLoadBalancerTest, EnabledLocalityWeightAwareness) {
   EXPECT_EQ(host_set_.healthy_hosts_per_locality_->get()[1][0], lb_->chooseHost(&context));
 }
 
+TEST_P(SubsetLoadBalancerTest, GaugesUpdatedOnDestroy) {
+  EXPECT_CALL(subset_info_, fallbackPolicy())
+      .WillRepeatedly(Return(envoy::api::v2::Cluster::LbSubsetConfig::ANY_ENDPOINT));
+
+  std::vector<std::set<std::string>> subset_keys = {{"version"}};
+  EXPECT_CALL(subset_info_, subsetKeys()).WillRepeatedly(ReturnRef(subset_keys));
+
+  init({
+      {"tcp://127.0.0.1:80", {{"version", "1.0"}}},
+  });
+
+  EXPECT_EQ(1U, stats_.lb_subsets_active_.value());
+  EXPECT_EQ(0U, stats_.lb_subsets_removed_.value());
+
+  lb_ = nullptr;
+
+  EXPECT_EQ(0U, stats_.lb_subsets_active_.value());
+  EXPECT_EQ(1U, stats_.lb_subsets_removed_.value());
+}
+
 INSTANTIATE_TEST_CASE_P(UpdateOrderings, SubsetLoadBalancerTest,
                         testing::ValuesIn({REMOVES_FIRST, SIMULTANEOUS}));