protected void doRequestDuration(RequestMetric metric) {
        requestDuration.labels(nullToEmpty(metric.getMethod(),
                Integer.toString(metric.getResponseCode()),
                metric.getApiId(),
                metric.getApiVersion(),
                metric.getClientId())).observe(metric.getRequestDuration());
    }
Prometheus scrape metrics.

Config:
<ul>
  <li>port - port for web-server to listen on for scrape requests</li>
</ul>

@author Marc Savy {@literal <msavy@redhat.com>}
gateway/engine/prometheus/src/main/java/io/apiman/gateway/engine/prometheus/PrometheusScrapeMetrics.java