Incrementing a counter from a Python subthread doesn't seem to do anything

For example:

```

from concurrent import futures
import threading

from absl import app
from absl import logging
import
apache_beam as beam

NAMESPACE = 'METRICS_THREADS_REPRO'

main_thread_counter = beam.metrics.Metrics.counter(
 
  NAMESPACE, 'main_thread_counter')
sub_thread_counter = beam.metrics.Metrics.counter(
    NAMESPACE,
'sub_thread_counter')

def increment_counter(counter):
  counter.inc()
  logging.info('Incremented
counter %s from thread %s',
               counter.metric_name, threading.current_thread().name)

class
IncrementCountersFn(beam.DoFn):

  def setup(self):
    self.executor = futures.ThreadPoolExecutor()

 
def process(self, idx):
    increment_counter(main_thread_counter)
    self.executor.submit(increment_counter,
sub_thread_counter).result()
    logging.info('Processed %i', idx)

def main(argv):
  if len(argv)
> 1:
    raise app.UsageError('Too many command-line arguments.')

  p = beam.Pipeline()
  _ = (
 
    p
      | 'Create' >> beam.Create(range(100))
      | 'Process' >> beam.ParDo(IncrementCountersFn()))

 
result = p.run()
  result.wait_until_finish()

  filter_by_namespace = beam.metrics.MetricsFilter().with_namespace(NAMESPACE)
 
filtered_metrics = result.metrics().query(filter_by_namespace)
  logging.info('Pipeline finished, metrics
logged: %s', filtered_metrics)

if __name__ == '__main__':
  app.run(main)

```

Only `main_thread_counter` is incremented, not `sub_thread_counter`:
```

I1203 18:38:56.394423 140078103397056 pipeline.py:48] Pipeline finished, metrics logged: {'counters':
[MetricResult(key=MetricKey(step=Process, metric=MetricName(namespace=METRICS_THREADS_REPRO, name=main_thread_
counter),
labels={}), committed=100, attempted=100)], 'distributions': [], 'gauges': []}

```

Imported from Jira [BEAM-13379](https://issues.apache.org/jira/browse/BEAM-13379). Original Jira may contain additional context.
Reported by: alimuldal.