message: http2: Add assert to MetadataEncoder.
  During MetadataEncoder::packNextFramePayload, assert that the payload size queue
  isn't empty before using it.
  Risk Level: Low
  Testing: bazel test //test/...
  Docs Changes: n/a
  Release Notes: n/a
diff: @@ -80,6 +80,7 @@ bool MetadataEncoder::createHeaderBlockUsingNghttp2(const MetadataMap& metadata_
  bool MetadataEncoder::hasNextFrame() { return payload_.length() > 0; }
  uint64_t MetadataEncoder::packNextFramePayload(uint8_t* buf, const size_t len) {
  + ASSERT(!payload_size_queue_.empty());
  const uint64_t current_payload_size =
  std::min(METADATA_MAX_PAYLOAD_SIZE, payload_size_queue_.front());
  