dataflow_metrics_test hard to maintain fake data

We have no way to dump and update the metric fake data this test uses. We should consider writing a tool to do this properly. We can change the format as long as its still human readable.

I tried and the following failed:

(1) Using json.dumps on the job_metrics

Fails for not being JSON serializable

(2) Using the proto json_format.py tools to dump on the job_metrics

Fails with exception: object has no attribute 'DESCRIPTOR'

https://github.com/googleapis/google-cloud-python/issues/3485 

(2) Using the proto text_format.py tools to dump on the job_metrics

Seemed to dump an empty file?

(3) Using this code to turn any object into a simple JSON style dict and print the JSON. This fails because it will print out all the private variables and other things we don't want. We could use this approach and allowlist the relevant fields.

def _create_simple_obj(self, obj):
  # If its a dictionary or defined class type
  if hasattr(obj, '__dict__') or isinstance(obj, dict):
    items = obj.items() if isinstance(obj, dict) else obj.__dict__.items()
    simple_dict = dict()
    for key, value in items:
      simple_dict[key] = self._create_simple_obj(value)
     return simple_dict
  elif isinstance(obj, (tuple, list, set, frozenset)):
    simple_list = []
    for x in obj:
      simple_list.append(self._create_simple_obj(x))
    return simple_list
  else:
    return obj

def _pretty_format_job_metrics(self, job_metrics):
  job_metrics = self._create_simple_obj(obj)
  return json.dumps(obj, indent=4, sort_keys=True)

Or we can just give up and not try to dump new ones, just add some basic tests to make sure it can convert the metrics format to MetricResults.

Then add more extensive testing on the MetricResults themselves with the matchers we are adding.