Ensure that all existing code handles cases of missing data with graceful partial failure

[joehandzik]

Essentially, avoid the same situation we ran into with weirdly missing data from the stats file.

[joehandzik]

@roclark I took a look at this yesterday and didn't see anything, expect for maybe in the brw_stats parser. Can you double check that and, if you think there's no problem, close out this issue? Specific line to start at (I think): https://github.com/HewlettPackard/lustre_exporter/blob/master/sources/procfs.go#L455

[roclark]

We are good on BRW stats due to how the metrics are written (we get all or nothing). Since the metricMap is allocated at the top of the function where we actually parse data, there isn't a concern about accessing invalid memory.