Add mountstats collector for detailed NFS statistics

[mdlayher]

First pass at a mountstats collector for detailed NFS statistics. There are ~25 more statistics I can currently add but I wanted to check in my work before doing a bunch more.

Fixes #366 .

[SuperQ]

woot

[mdlayher]

Awesome. @SuperQ, do you want me to tack more changes onto this PR, or make a new one for the additional stats?

[SuperQ]

No, I think we should do this incrementally. I'd like to get one other thumb from someone, maybe @beorn7 or @grobie.

[beorn7]

Looks good in general, just the confusion about the "items in queue" metrics, if they are Gauges or Counters. (If the latter, the help string should be reworded, something like "total number of items added to the queue" or something.)

[beorn7]

This sounds like it could go down, i.e. it should be a Gauge?

[beorn7]

Same as above, help string suggests a Gauge.

[beorn7]

As above.

[mdlayher]

@beorn7 good call. According the docs I followed while parsing these stats:

cumulative backlog count: Every time we send a request, we add the current backlog queue size to this counter.
...
cumulative sending queue count: Every time we send a request, we add the current size of the sending queue to this counter.
cumulative pending queue count: Every time we send a request, we add the current size of the pending queue to this counter.

Based on these descriptions, I believe your suggestion of "total number of items added to the queue" will be adequate.

[mdlayher]

@beorn7 Updated. Please take a look at your convenience.

[beorn7]

In lack of more qualified reviewers, I'll approve and merge.