Method protocol

src/brpc/span.h:173–173

    const butil::EndPoint& remote_side() const { return _remote_side; }
    SpanType type() const { return _type; }
    ProtocolType protocol() const { return _protocol; }
    int error_code() const { return _error_code; }
    int request_size() const { return _request_size; }
    int response_size() const { return _response_size; }