private synchronized void metricDelayedResponses(ApplicationId app, int elems) {
        metrics.computeIfAbsent(app, key -> rpcServer.metricUpdaterFactory()
                                                     .getOrCreateMetricUpdater(Metrics.createDimensions(key)))
               .setDelayedResponses(elems);
    }
Creates a DelayedConfigResponse object for taking care of requests that should
not be responded to right away.  Puts the object on the delayedResponsesQueue.

NOTE: This method is called from multiple threads, so everything here needs to be
thread safe!

@param request a JRTConfigRequest
configserver/src/main/java/com/yahoo/vespa/config/server/rpc/DelayedConfigResponses.java