enhance VolumeStats object with new fields

[glimchb]

Consider add:

• Flush commands count in addition to Read/Write/Unmap
• Admin commands count in addition to Read/Write/Unmap
• Error count to every Read/Write/Unmap/Flush/Admin Ops
• Async event count
• change from int32 to int64 (comment from @llabordehpe on [storage] define statistics object #220)

@benlwalker @llabordehpe @mkalderon @jainvipin @GottliebNoam @orendu WDYT ?

Some of them applicable to FrontEnd only sand not BackEnd volumes, I think...

see https://github.com/opiproject/opi-marvell-bridge/blob/main/spdk.go#L287 - MrvlNvmGetCtrlrStatsResult
see https://github.com/opiproject/opi-nvidia-bridge/blob/main/spdk.go#L115 - NvdaControllerNvmeStatsResult

[jainvipin]

+1 on the idea of adding more stats; how do we make the user aware that some implementation is not supporting all the stats e.g. do we keep them zero? I hope not all stats are mandatory - if not, how do we identify optional and mandatory stats.

[glimchb]

@jainvipin what from the existing stats or proposed here, you feel should be optional ?

[mkalderon]

LGTM