Cleanup metrics/counters code

[kennknowles]

E.g. right now we have

metricbase.Distribution
metric.DelegatingDistribution
metric.cells.DistributionCell
metric.cells.DistributionResult
metric.cells.DistributionData
metric.cells.DistributionAggregator
transforms.cy_combiners.DataflowDistributionCounter
transforms.cy_combiners.DataflowDistributionCounterFn
transforms.cy_dataflow_distribution_counter.DataflowDistributionCounter

plus some code under util/counters. This is true for "ordinary" sum and max counters as well. We should consolidate/simplify this code. 

 

Imported from Jira BEAM-4074. Original Jira may contain additional context.
Reported by: robertwb.