@Nullable
    private static String caffeineSpec(Function<FlagsProvider, String> method, String flagName,
                                       boolean allowOff) {
        final String spec = getValue(method, flagName, value -> {
            try {
                if ("off".equals(value)) {
                    return allowOff;
                }
                CaffeineSpec.parse(value);
                return true;
            } catch (Exception e) {
                return false;
            }
        });

        if (!"off".equals(spec)) {
            return spec;
        }

        if (allowOff) {
            return null;
        }

        // We specified 'off' as the default value for the flag which can't be 'off'.
        throw new Error();
    }
Returns whether {@link AnnotatedService} should leave request/response content logs
by default when a {@link LoggingService} is added.

<p>By default, this option is enabled. Specify the
{@code -Dcom.linecorp.armeria.annotatedServiceContentLogging=false} JVM option to
override the default value.
core/src/main/java/com/linecorp/armeria/common/Flags.java