private void reportMetricsUnknown(RoutingTable.Target target) {
        var dimensions = metricsDimensionsForService(target);
        var context = metric.createContext(dimensions);
        metric.set(UPSTREAM_UP_METRIC, 0L, context);
        metric.set(UPSTREAM_DOWN_METRIC, 0L, context);
        metric.set(UPSTREAM_UNKNOWN_METRIC, 1L, context);
    }
Report Nginx metrics periodically.

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