Remove gauges on connection pool manager shutdown

Fixes #47
This test is actually testing for a few different things: 

1. Closing and recreating a client based on a name
2. Checking that upon closing, the gauges for a client are destroyed/removed from the registry
3. Checking in the registry to make sure the gauges for a client can be found after creation and before closing
4. Inherently checking that `#registryName` properly generates a string that can be used to lookup the gauges for a client.

 In the interest of using this spec as a spec and easily knowing what went wrong during the running of tests, it would be nice either update the title of this test to reflect everything it's testing for or splitting each/some of the above into separate tests. I would prefer the latter.