QuotaSpec's match rule not support StringMap value type

Policies and Telemetry
1 
apiVersion: config.istio.io/v1alpha2
kind: QuotaSpec
metadata:
  name: quotaspec
  namespace: istio-system
spec:
  rules:
  - match:
    - clause:
        request.headers["user"]:
          exact: raft
    quotas:
    - charge: 1
      quota:  requestquota

I want to use QuotaSpec as above, only matching request which http header has user and its value exact “raft”.

But It dosen’t work.
After reading code, I found that

// istio-proxy/src/istio/quota_config/config_parser_impl.cc
bool ConfigParserImpl::MatchAttributes(const AttributeMatch& match,
                                       const Attributes& attributes) const {
  const auto& attributes_map = attributes.attributes();
  for (const auto& map_it : match.clause()) {
    // map is attribute_name to StringMatch.
    const std::string& name = map_it.first;
    const auto& match = map_it.second;

    // Check if required attribure exists with string type.
    const auto& it = attributes_map.find(name);
    if (it == attributes_map.end() ||
        it->second.value_case() != Attributes_AttributeValue::kStringValue) {
      return false;
    }
    const std::string& value = it->second.string_value();
    ...
}

It only support string type，so why not support stringmap? Are there any other considerations?