Add partition stats for folder mounted volumes in Windows to monitor filesystems

[bjsignalfx]

Made changes to collect partitions stats for partitions in volumes mounted to drives and folders in Windows.

• Tested in Windows Server 2016
• Compared against gopsutil v3.10.20

[rmfitzpatrick]

Would you be able to add unit and/or windows marked integration tests?

[bjsignalfx]

@rmfitzpatrick writing the integration test seems to be more involved than I thought. So, I asked @jchengsfx for access to his EC2 windows instance (ec2-35-163-38-92.us-west-2.compute.amazonaws.com) where I created a 100 MiB VHD and mounted it to C:\mnt\testHD. I'm running the partitions monitor there and you can see the data here.

You can search for the other filesystems metrics using filesystems-bremer.

[rmfitzpatrick]

@bjsignalfx, it's pretty standard to hardcode a historic view or return value while writing unit tests to ensure that they result in the expected structures/values for things that are nontrivial to mock or that require cumbersome integration test environments. Having a "works for me" example at PR time doesn't help detect regressions or provide helpful scaffolding for bug fixes and enhancements in the future.

[bjsignalfx]

@rmfitzpatrick I added tests to compare the functionality against gopsutils. Crucially there is no test for getting partition stats of folder mounts which is what this fix is for.