private Object chooseLevel(final Optional<String> levelString) {
        switch (levelString.orElse("info")) {
            case "error":
                return this.ERROR;
            case "warn":
                return this.WARN;
            case "info":
                return this.INFO;
            case "debug":
                return this.DEBUG;
            case "trace":
                return this.TRACE;
            default:
                throw new IllegalArgumentException(String.format(
                        "Log level %s is invalid. Available levels are error, warn, info, debug and trace.",
                        levelString.orElse("(null)")));
        }
    }
Configures Logback's root logger.

<p>It calls Logback's configurators through reflection so that embulk-core does not need to depend on Logback's classes.
embulk-core/src/main/java/org/embulk/cli/CliLogbackConfigurator.java