Count combiner does not work as expected

[anwarfg]

Is there an existing issue for this?

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Current Behavior

Currently takes two floats, rather than a SumPartial. This deviates from the expected behaviour of the sum combiner.

Expected Behavior

The count combiner does not behave like the sum combiner. An issue in some examples has led me to find that the count combiner takes floats and adds them, rather than taking a sumPartial and returning the count.

This line:
count_combiner = Combiner[float, float](finalise=lambda x: x)

should probably be
count_combiner = Combiner[SumPartial, float](finalise=lambda x: x.count)

This would retain the same structure as the sum_combiner, and be used in a similar way.

Steps To Reproduce

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