private void exportBuiltIns(State state, RequestLog log) {
        if (builtInPropertyArray != null) {
            for (final ExportEntry<BuiltInProperty> entry : builtInPropertyArray) {
                final String value = entry.key.converter.apply(log);
                if (value != null) {
                    state.put(entry.exportKey, value);
                }
            }
        }
    }
Returns a {@link Map} whose key is an export key set through {@code add*()} in
{@link RequestContextExporterBuilder} and value is extracted from the specified {@link RequestContext}.
core/src/main/java/com/linecorp/armeria/common/logging/RequestContextExporter.java