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.
This is a re-architect of the vmpooler initialisation code to:
1. Allow an API service for both manager and the api
2. Add the Prometheus endpoints to the web service.
Needed to change the way the Rack Service is started as instantiating
using ".New" leads to a failure to initialise the http Stats
collection.
3. Selectively load the pooler api and/or Prometheus endpoints.
4. Rework API Spec tests for revised API loading. Needed to tidy up the
initialisation and perform a reset! after each test to avoid "leaks"
and dependencies between the tests.
This commit adds a capability to vmpooler to reset a pool, deleting its ready and pending instances and replacing them with fresh ones. Without this change vmpooler does not offer a mechanism to reset a pool without also changing its template.
