VMware collector support for units and value modifier

The raw metrics provided from the VMware API can't be used directly. Each measurement has meta information such as a value modifier and the unit. To collect the metrics correctly the value modifier and the unit need to be collected as well and taken into account when stored in the timer series storage in OpenNMS to be useful.

See https://www.vmware.com/support/developer/converter-sdk/conv61_apireference/vim.PerformanceManager.html