message: use initializer list for constructor
diff: @@ -572,9 +572,7 @@ public:
  }
  }
  }
  - ResponseHeader(const MessageMetadata& metadata) {
  - spans_ = metadata.spans();
  -
  + ResponseHeader(const MessageMetadata& metadata) : spans_(metadata.spans()) {
  metadata.headers().iterate(
  [](const Http::HeaderEntry& header, void* cb) -> Http::HeaderMap::Iterate {
  absl::string_view key = header.key().getStringView();
  