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.
