CP-8866:Develop for Resource Statistic Feature on XenCenter

Based on the requirements draft and discussed solution, develop the prototype for Resource Statistic Feature part on XenCenter, that is, provide the export to Excel and CSV functionality with required data sets,by accessing the XenCenter cache.
Signed-off-by: Cheng Zhang <cheng.zhang@citrix.com>
In the section starting here, there is a lot of code duplicated from PropertyAccessorHelper. Having two copies of the same code which might drift apart is risky, so it would be better to use the version there, which is public and static. This would mean using the MetricUpdater from that class, and not instantiating another one here. (You could still use the UpdateOnce() function from there.)