HDDS-10467. Reduce metrics visibility

[adoroszlai]

What changes were proposed in this pull request?

Metrics sources should try to avoid exposing mutable metric objects as public.

• reduce visibility for most methods
• change some to return value() instead of the mutable metric itself
• move OzoneManagerDoubleBufferMetrics to the package where OzoneManagerDoubleBuffer is

https://issues.apache.org/jira/browse/HDDS-10467

How was this patch tested?

CI:
https://github.com/adoroszlai/ozone/actions/runs/8160490612

[myskov]

As tests for this class are in the same package, the VisibleForTesting annotation can be also removed.

[adoroszlai]

I also considered removing the annotation. Here's why I kept it (so far):

Javadoc of @VisibleForTesting says it should not be used on public or protected members, since that indicates bad design. By that logic it should only be applied to members with package-private (default) visibility. So removing it when tests are in the same package means we would never use the annotation.

On the other hand, I'm not sure if it's worth sprinkling all over the code for trivial accessors. Maybe we should limit it to methods with actual logic. Such methods exist in their own right for the class itself, only visibility is affected by usage in tests.

[myskov]

VisibleForTesting annotation is overused in the codebase. I think a ticket named "Reduce metrics visibility" is a good chance to put things in order there, including the annotation.
If removing an annotation doesn't break tests, the annotation is not needed and should be removed.

[myskov]

Thanks for updating the patch, LGTM @adoroszlai

[adoroszlai]

Thanks @kerneltime, @myskov for the review.