Python SDK: Collect metrics in non-cython environment

With the portable Flink runner, the metric is reported as 0, while the count metric works as expected.

https://lists.apache.org/thread.html/25eec8104bda6e4c71cc6c5e9864c335833c3ae2afe225d372479f30@%3Cdev.beam.apache.org%3E

 Edit: metrics are collected properly when using cython, but not without cython. This is because state sampling has yet to be implemented in a way that does not depend on cython [1].

[1] https://github.com/apache/beam/blob/master/sdks/python/apache_beam/runners/worker/statesampler_slow.py#L62