Switch to spdlog for Logging Instead of cout

[thomaskleiven]

As suggested in your comments #L22, it would be great to switch to spdlog instead of using cout for logging.

I can put up a PR for this if it is of interest.

[cyrusbehr]

Yes please! Would also be valuable if you use an environment variable to control the log level: ex.

 std::string logLevelStr(envValue);
    if (logLevelStr == "trace") {
        return spdlog::level::trace;
    } else if (logLevelStr == "debug") {
        return spdlog::level::debug;
    } else if (logLevelStr == "info") {
        return spdlog::level::info;
    } else if (logLevelStr == "warn") {
        return spdlog::level::warn;
    } else if (logLevelStr == "err" || logLevelStr == "error") {
        return spdlog::level::err;
    } else if (logLevelStr == "critical") {
        return spdlog::level::critical;
    } else if (logLevelStr == "off") {
        return spdlog::level::off;
    } else {
        std::cerr << "Unknown log level: " << logLevelStr << ". Using default log level (info)." << std::endl;
        return spdlog::level::info;
    }

Please also add to readme explaining how the user would set the log level.