Added new constructors to HealthCheckServlet and MetricsServlet

Making it possible to assign registries via constructor rather than via
ServletConfig.
Useful for programmatic embedding and test scenarios.
Adhere to coding standard
Added some more "final"s in Test cases
Manually merge upstream into pullrequest 438
Merge pull request #438 from jhberges/master

Assign registries to HealthCheck/Metrics-Servlets via constructor