Add support for unique metric to facet streaming function

Add support for Unique metric to facet function which under the cover utilizes JSON Facet API.

The challenge is to come up with a keyword which can be used for UniqueMetric. Currently "unique" is used for UniqueStream and can't be utilized.

Does "uniq" make sense?

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StreamFactory factory = new StreamFactory()
.withCollectionZkHost (...)
.withFunctionName("facet", FacetStream.class)
.withFunctionName("sum", SumMetric.class)
.withFunctionName("unique", UniqueStream.class)
.withFunctionName("unique", UniqueMetric.class)
...
...