message: tracing: Added tests for casting to HttpTracerImpl.
  Follow-up to adding tests for being able to cast the tracer to HttpTracerImpl when zipkin is configured.
diff: @@ -149,6 +149,9 @@ protected:
  EXPECT_TRUE(server_->api().fileExists("/dev/null"));
  }
  + // Returns the server's tracer as a pointer, for use in dynamic_cast tests.
  + Tracing::HttpTracer* tracer() { return &server_->httpContext().tracer(); };
  +
  Network::Address::IpVersion version_;
  testing::NiceMock<MockOptions> options_;
  DefaultTestHooks hooks_;
  @@ -382,14 +385,22 @@ TEST_P(ServerInstanceImplTest, NoHttpTracing) {
  options_.service_cluster_name_ = "some_cluster_name";
  options_.service_node_name_ = "some_node_name";
  EXPECT_NO_THROW(initialize("test/server/empty_bootstrap.yaml"));
  - EXPECT_NE(nullptr, dynamic_cast<Tracing::HttpNullTracer*>(&server_->httpContext().tracer()));
  + EXPECT_NE(nullptr, dynamic_cast<Tracing::HttpNullTracer*>(tracer()));
  + EXPECT_EQ(nullptr, dynamic_cast<Tracing::HttpTracerImpl*>(tracer()));
  }
  TEST_P(ServerInstanceImplTest, ZipkinHttpTracingEnabled) {
  options_.service_cluster_name_ = "some_cluster_name";
  options_.service_node_name_ = "some_node_name";
  EXPECT_NO_THROW(initialize("test/server/zipkin_tracing.yaml"));
  - EXPECT_EQ(nullptr, dynamic_cast<Tracing::HttpNullTracer*>(&server_->httpContext().tracer()));
  + EXPECT_EQ(nullptr, dynamic_cast<Tracing::HttpNullTracer*>(tracer()));
  +
  + // Note: there is no ZipkingTracerImpl object;
  + // source/extensions/tracers/zipkin/config.cc instantiates the tracer with
  + // std::make_unique<Tracing::HttpTracerImpl>(std::move(zipkin_driver), server.localInfo());
  + // so we look for a successful dynamic cast to HttpTracerImpl, rather
  + // than HttpNullTracer.
  + EXPECT_NE(nullptr, dynamic_cast<Tracing::HttpTracerImpl*>(tracer()));
  }
  } // namespace Server
  