Add some minimal tests to metrics

[mogren]

What type of PR is this?
cleanup

Which issue does this PR fix:

• Fix broken mocks
• Add generate_mock.go for discovery.go
• Initial scaffolding to add more unit tests (discovery.go needs a cleanup..)
• Ignore main.go for test coverage, it is just for wiring together the other components.

Why do we need it:
Small steps to clean up the code base

Testing done on this change:
Just added a unit test, and make unit-test is passing

Automation added to e2e:
No

Will this break upgrades or downgrades. Has updating a running cluster been tested?:
No

Does this change require updates to the CNI daemonset config files to work?:
No

Does this PR introduce any user-facing change?:
No

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.