printing non-utf8 string from metadata crashes Envoy (when trace enabled)

[stevenzzzz]

Title: printing non-utf8 string from metadata crashes Envoy (when trace enabled)

Description:
envoy/http/metadata_interface.h seems assume that metadata K:V are in utf8 strings：

class MetadataMap : public UnorderedStringMap {
public:
  using UnorderedStringMap::UnorderedStringMap;

  friend std::ostream& operator<<(std::ostream& out, const MetadataMap& metadata_map) {
    out << "metadata map:";
    for (const auto& metadata : metadata_map) {
      out << "\nkey: " << metadata.first << ", value: " << metadata.second << std::endl;
    }
    return out;
  }
};

But seems no such constraint in spec for metadata.

We should at least eacape the K:V before passing them into spdglog which expects utf8 string.
https://github.com/abseil/abseil-cpp/blob/master/absl/strings/escaping.h

Repro steps:

1. Enable trace logging in Envoy,
2. put some protobuf::serializeAsString() as stream metadata value.

Envoy will crash when the serializeAsString() contains invalid utf8 string, while spdlog tries to interpret the value as an utf8 string.

[stevenzzzz]

assign to @yanavlasov for triaging.
/assign @yanavlasov

[stevenzzzz]

cc @adisuissa

[stevenzzzz]

/assign stevenzzzz

[github-actions]

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[stevenzzzz]

the fix is in.