All Python counters should use the same code path

Right now we have Metrics.counter, Metrics.distribution counters which use MetricContainers to store and accumulate metrics.

CounterSet counters which use accumulators to store and accumulate metrics. This code path is optimized using cython. 
See cy_combiners.py

Ideally we can keep the user interface (this should not change) for creating metrics with Metrics.counter(), .etc. But use the underlying optimized CounterSets.