private void reportConfigAge() {
        Path temporaryNginxConfiguration = NginxPath.temporaryConfig.in(fileSystem);
        Path nginxConfiguration = NginxPath.config.in(fileSystem);
        Optional<Instant> temporaryConfigModified = lastModified(temporaryNginxConfiguration);
        if (temporaryConfigModified.isEmpty()) {
            metric.set(CONFIG_AGE_METRIC, 0, metric.createContext(Map.of()));
            metric.set(CONFIG_RELOAD_TIME, 0, metric.createContext(Map.of()));
            return;
        }
        Instant configModified = lastModified(nginxConfiguration).orElse(Instant.EPOCH);
        long secondsDiff = Math.abs(Duration.between(configModified, temporaryConfigModified.get()).toSeconds());
        long reloadTime = Math.abs(Duration.between(temporaryConfigModified.get(), Instant.now()).toSeconds());
        metric.set(CONFIG_AGE_METRIC, secondsDiff, metric.createContext(Map.of()));
        metric.set(CONFIG_RELOAD_TIME, reloadTime, metric.createContext(Map.of()));
    }
Report Nginx metrics periodically.

@author Morten Tokle
@author Martin Polden
routing-generator/src/main/java/com/yahoo/vespa/hosted/routing/nginx/NginxMetricsReporter.java