Cleanup metric names and types

[SuperQ]

It looks like a number of metric names are copies of the raw memcached stats names. These don't follow naming best practices and many likely need some cleanup work.

Also, it seems like a number of metrics are being exposed as counters, when some of them are gauges.

[elukey]

Definitely interested in working on this, I am packaging a new version of the exporter so I'd prefer to get stable metric names/types (if we change any of them) :)