[aggregator] Adding cardinality by source metrics

[MGaulia]

What this PR does / why we need it:
We want cardinality metrics coming from aggregator, this is done at the ticker and the metrics get pushed when the flush happens.

I added possibility to disable the metrics and option to set your own source tag (by default source will be collected from the service tag)

Does this PR introduce a user-facing and/or backwards incompatible change?:

Add per flush cardinality metrics

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