private void exportRequestHeaders(State state, RequestLog log) {
        if (reqHeaders == null || !log.isAvailable(RequestLogProperty.REQUEST_HEADERS)) {
            return;
        }

        exportHeaders(state, log.requestHeaders(), reqHeaders);
    }
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