Expose amount of shuffle data available on the node

ShuffleMetrics doesn't have metrics like 
"totalShuffleDataBytes" and "numAppsWithShuffleData", these metrics are per node published by External Shuffle Service.
 
Adding these metrics would help in -
1. Deciding if we can decommission the node if no shuffle data present
2. Better live monitoring of customer's workload to see if there is skewed shuffle data present on the node