Introducing the Prometheus Stats code into ABS showed that the Clarity
could be improved a bit with better variable naming, some refactoring
to reduce repitition and documenting the Metrics table itself.
Filtering these changes back to the vmpooler code base.
Ensure that the correct stats are registered for the Manager and the api
respectively. E.g. all checkout counters are for the api only, whereas
clone times belong to the manager.
Also new ondemand functionality stats weren't registered, so add these
along with missing delete stats.
Review changes suggested to revise the Metrics related files into a more
logical class structure.
Also fixup grammar typos in docs strings and any trailing metrics that
have been recently added to vmpooler.
Break down the usage stats into smaller groups so as to manage the
number of stat lines collected for Prometheus.
This may need some further revision to filter out Litmus stats, or
otherwise collect litmus usage information.
Add a new Prometheus class as an additional stats feed along with the
existing feeds.
Move the metrics initialisation code into its own class and sub-class
the individual metrics implementations under this.
