Add metric jdisc.http.request.requests_per_connection (#36615)

Author: bjormel

metrics/src/main/java/ai/vespa/metrics/set/Vespa9VespaMetricSet.java

@@ -158,6 +158,7 @@ private static Set<Metric> getContainerMetrics() {
 
         addMetric(metrics, ContainerMetrics.JDISC_HTTP_REQUEST_PREMATURELY_CLOSED.rate());
         addMetric(metrics, ContainerMetrics.JDISC_HTTP_REQUEST_CONTENT_SIZE, EnumSet.of(sum, count, max));
+        addMetric(metrics, ContainerMetrics.JDISC_HTTP_REQUEST_REQUESTS_PER_CONNECTION, EnumSet.of(sum, count, max));
         addMetric(metrics, ContainerMetrics.JDISC_HTTP_REQUESTS.count());
 
         addMetric(metrics, ContainerMetrics.JDISC_HTTP_SSL_HANDSHAKE_FAILURE_EXPIRED_CLIENT_CERT.rate());