message: Use correct protobuf string type
  For string fields extracted from a map, use the correct string type.
  Risk Level: low
  Testing: built the affected target
  Docs Changes: n/a
  Release Notes: n/a
diff: @@ -105,7 +105,7 @@ JsonTranscoderConfig::JsonTranscoderConfig(
  }
  PathMatcherBuilder<const Protobuf::MethodDescriptor*> pmb;
  - std::set<std::string> ignored_query_parameters;
  + std::set<ProtobufTypes::String> ignored_query_parameters;
  for (const auto& query_param : proto_config.ignored_query_parameters()) {
  ignored_query_parameters.insert(query_param);
  }
  