message: formatter: use absl instead of std
diff: @@ -203,7 +203,7 @@ ProtobufWkt::Struct JsonFormatterImpl::toStruct(const Http::RequestHeaderMap& re
  auto* fields = output.mutable_fields();
  JsonFormatMapVisitor visitor{json_format_map_callback, providers_callback};
  for (const auto& pair : *format.value_) {
  - ProtobufWkt::Value value = std::visit(visitor, pair.second);
  + ProtobufWkt::Value value = absl::visit(visitor, pair.second);
  if (omit_empty_values_ && value.kind_case() == ProtobufWkt::Value::kNullValue) {
  continue;
  }
  