4.x Make RegistryFactory and its getInstance and getRegistry methods public

[tjquinno]

Description

Resolves #8161

In 3.x, the RegistryFactory class was public in the metrics API module and usable from both SE and MP. It had public methods getInstance() and getRegistry() and was injectable.

In 4.x, we moved it to MP because it no longer applied in SE with the new neutral metrics API. At that time, I also made the class package-private because RegistryFactory only had a role in MP metrics now, and I anticipated that users would @Inject MetricRegistry rather than work with the RegistryFactory.

The 4.x doc still referred to RegistryFactory and, in fact, it is still a valid use case to either @Inject the RegistryFactory or retrieve it programmatically using a static method so as to invoke its getRegistry method to locate a MetricRegistry programmatically. As we found out in our internal Slack channel, users had used these approaches in 3.x and we broke their code by removing this capability.

Restoring these features, essentially to how they worked in 3.x, enlarges our surface area only very slightly as compared to 4.0.1 and allows existing user code to continue to work with only a change in import statements (because RegistryFactory is in a different package in 4.x).

This PR does the following:

1. Make RegistryFactory and its getInstance() and getRegistry(String) methods public.
   1. Change the return type of getRegistry from Registry (a package-private class) to MetricRegistry (an MP metrics interface which Registry implements).
   2. Add a package-private method registry(String) to RegistryFactory which returns a Registry for use by our own methods in the package.
2. Update the MP metrics doc to clarify and show how to get and use a RegistryFactory instance.

Documentation

Changes included in the PR.

[ljamen]

Small doc updates - LGTM.