public Map<String, String> export() {
        final RequestContext ctx = RequestContext.currentOrNull();
        return ctx != null ? export(ctx) : ImmutableMap.of();
    }
Returns a {@link Map} whose key is an export key set through {@code add*()} in
{@link RequestContextExporterBuilder} and value is extracted from {@link RequestContext}.
Note that this method returns an empty {@link Map} if current {@link RequestContext} is {@code null}.
core/src/main/java/com/linecorp/armeria/common/logging/RequestContextExporter.java