message: buffer filter: Deflake RouterRequestPopulateContentLength
diff: @@ -51,12 +51,16 @@ TEST_P(BufferIntegrationTest, RouterRequestPopulateContentLength) {
  ASSERT_TRUE(fake_upstreams_[0]->waitForHttpConnection(*dispatcher_, fake_upstream_connection_));
  ASSERT_TRUE(fake_upstream_connection_->waitForNewStream(*dispatcher_, upstream_request_));
  + upstream_request_->encodeHeaders(default_response_headers_, true);
  ASSERT_TRUE(upstream_request_->waitForEndStream(*dispatcher_));
  +
  auto* content_length = upstream_request_->headers().ContentLength();
  ASSERT_NE(content_length, nullptr);
  EXPECT_EQ(content_length->value().getStringView(), "9");
  +
  response->waitForEndStream();
  ASSERT_TRUE(response->complete());
  + EXPECT_EQ("200", response->headers().Status()->value().getStringView());
  }
  TEST_P(BufferIntegrationTest, RouterRequestBufferLimitExceeded) {
  