TLDR:
logger.log( logfmtxx::level::error, "internal server error", logfmtxx::field{"http.method", "GET"}, logfmtxx::field{"http.path", "/"}, logfmtxx::field{"http.status", 500} );
This is a very dated approach to structural logging.
Modern implementations are effortless: https://izumi.7mind.io/logstage/index.html