Sources using Metrics.counter fail with 'Can't pickle ... DelegatingCounter'

This fails under Python 2.7 but works fine under Python 3 – looks like an easy fix.

I am reading from a beam.io.utils.CountingSource:

`beam.io.Read(beam_io_utils.CountingSource(NUM_ROWS))`

This fails under python 2.7 with:

Python exception: Can't pickle : attribute lookup apache_beam.metrics.metric.DelegatingCounter failed

Looks like this is because DelegatingCounter is actually an inner class defined inside apache_beam.metrics.metric.Metric. Under Python 3 pickling can resolve it under its qualified name apache_beam.metrics.metric.Metric.DelegatingCounter, but not in Python 2. A fix would be to move it to a top-level class in that module.

This also applies to other sources which use Metrics.counter.

Imported from Jira [BEAM-6389](https://issues.apache.org/jira/browse/BEAM-6389). Original Jira may contain additional context.
Reported by: matthjw.