@VisibleForTesting
    RequestContextExporter exporter() {
        checkState(exporter != null);
        return exporter;
    }
A <a href="https://logback.qos.ch/">Logback</a> {@link Appender} that exports the properties of the current
{@link RequestContext} to {@link MDC}.

<p>Read '<a href="https://armeria.dev/docs/advanced-logging">Logging contextual information</a>'
for more information.
logback/logback12/src/main/java/com/linecorp/armeria/common/logback/RequestContextExportingAppender.java