Fix case when GC is disabled; Add more safeties

[pglombardo]

If the Python garbage collector is disabled, we were assuming the initialisation of a variable causing the following:

Traceback (most recent call last):
  File "/Users/pglombardo/.pyenv/versions/3.7.2/lib/python3.7/threading.py", line 917, in _bootstrap_inner
    self.run()
  File "/Users/pglombardo/.pyenv/versions/3.7.2/lib/python3.7/threading.py", line 865, in run
    self._target(*self._args, **self._kwargs)
  File "/Users/pglombardo/Projects/instana/python-sensor/instana/meter.py", line 149, in collect_and_report
    self.process()
  File "/Users/pglombardo/Projects/instana/python-sensor/instana/meter.py", line 161, in process
    cm = self.collect_metrics()
  File "/Users/pglombardo/Projects/instana/python-sensor/instana/meter.py", line 310, in collect_metrics
    gc=g)
UnboundLocalError: local variable 'g' referenced before assignment

This PR fixes the logic when the GC is disabled and adds more safety to that function (collect_metrics);