long getDiskCountOnNode(String instanceName) {
    InstanceConfig instanceConfig = instanceNameToInstanceConfigs.get(instanceName);
    return instanceConfig.getRecord().getMapFields().size();
  }
@param instanceName the instance on which this operation is being done.
@return the count of disks registered for the given node via this admin.
ambry-clustermap/src/test/java/com/github/ambry/clustermap/MockHelixAdmin.java