Do not expose innards of QueueMetrics object into FSLeafQueue#computeMaxAMResource

This is a follow-up for YARN-9323, covering required changes as discussed with templedf earlier.
After YARN-9323, org.apache.hadoop.yarn.server.resourcemanager.scheduler.fair.FSLeafQueue#computeMaxAMResource gets the QueueMetricsForCustomResources object from scheduler.getRootQueueMetrics().
Instead, we should use a "fill-in" method in QueueMetrics that receives a Resource and fills in custom resource values if they are non-zero.