FlinkFlowStepStats.getCountersFor should return counter names, not groupCounter names

[kkrugler]

Currently it returns the entire <group name> -> <key name> string, but callers expect to get back just the counter name (which they will then combine with the group name).

The current result is that trying to get counters for a completed Flow will return 0.

I've made a change and confirmed it fixes the issue, but need to add a test and put that all in a branch for a pull request.

[fhueske]

Thanks for reporting @kkrugler.
Looking forward to the PR. Once the issue is fixed, we can push out a 0.3 release.

[kkrugler]

Pull request created (#63).

[fhueske]

Pull request was merged.