private static void exportHeaders(State state, HttpHeaders headers,
                                      ExportEntry<AsciiString>[] requiredHeaderNames) {
        for (ExportEntry<AsciiString> e : requiredHeaderNames) {
            putStringifiedProperty(state, e, headers.get(e.key));
        }
    }
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