collect lustre rpc stats

[baallan]

Lustre has rpc data in files:

/proc/fs/lustre/mdc/*-mdc-*/rpc_stats
/proc/fs/lustre/osc/*-osc-*/rpc_stats

particularly of interest are

read RPCs in flight
write RPCs in flight
pending write pages
pending read pages
modify_RPCs_in_flight

though the histograms in the same files are not of interest for periodic collection.
The in flight numbers give some indication of what servers are highly loaded, even if (or especially if?) token bucket filters are active and prevent high byte count/operation count from being visible.
We need to think about the best way to create a sampler for these metrics.

[baallan]

@morrone do you have any thoughts on this issue? in particular we're looking for metrics that would provide evidence (from the client side) of lustre delays induced by token bucket filter activity.