message: event: fix DispatcherImplTest::InitializeStats flake
  Fix DispatcherImplTest::InitializeStats flake due to dispatcher lifetime exceeding mock stats scope lifetime.
  Risk Level: low
  Testing: bazel test //test/common/event:dispatcher_impl_test --runs_per_test=1000 (with ASAN enabled)
  Docs Changes: n/a
  Release Notes: n/a
  Fixes
diff: @@ -85,6 +85,7 @@ protected:
  dispatcher_thread_->join();
  }
  + NiceMock<Stats::MockStore> scope_; // Used in InitializeStats, must outlive dispatcher_->exit().
  Api::ApiPtr api_;
  Thread::ThreadPtr dispatcher_thread_;
  DispatcherPtr dispatcher_;
  @@ -96,11 +97,9 @@ protected:
  };
  TEST_F(DispatcherImplTest, InitializeStats) {
  - // NiceMock because deliverHistogramToSinks may or may not be called, depending on timing.
  - NiceMock<Stats::MockStore> scope;
  - EXPECT_CALL(scope, histogram("test.dispatcher.loop_duration_us"));
  - EXPECT_CALL(scope, histogram("test.dispatcher.poll_delay_us"));
  - dispatcher_->initializeStats(scope, "test.");
  + EXPECT_CALL(scope_, histogram("test.dispatcher.loop_duration_us"));
  + EXPECT_CALL(scope_, histogram("test.dispatcher.poll_delay_us"));
  + dispatcher_->initializeStats(scope_, "test.");
  }
  TEST_F(DispatcherImplTest, Post) {
  