Add CardinalityCounter to testmertics

[Vamsi-Mundra]

Reasons For the Change:

While we are using testmetrics pkg in one of our unit tests, we observed CheckCardinalityCounter is not working as expected.
Inside the Provider of testmetrics, all the counters we are using, are of metrics.go except for cardCounters. For cardCounters we are using xmetrics.HLLCounter. We weren't adding labels properly and CheckCardinalityCounter was not taking labelValues. Also all the counters of metrics.go are having Provider of testmetrics which is not for xmetrics.HLLCounter. We wanted to add new CardinalityCounter inside metrics.go of testmetrics that has Provider inside it and adds, checks for labels correctly.

Changes:

• Added CardinalityCounter which implements CardinalityCounter interface of go-kit/metrics
• Updated Provider to use CardinalityCounter of testmetrics

[ypaq]

Couple of comments, but looks good overall.

[ypaq]

LGTM